So looking at the maps and wiki pages, and I can't tell a clear answer to this - how close is the covenant to one or another of the great forests in the Rhine Tribunal?
In particular, if a magus wanted to build a lab "within the forest", would it be doable within the bounds of the covenant (perhaps but not necessarily in the caves)?
It all depends on what you mean by "within the forest". According to the map from GotF, the Black Forest extends down quite close to Basel, which is about 40 miles from the covenant.
But the Black Forest is clearly outside of the bounds of the covenant.
There are wooded areas withing the covenant's small valley (everything with a light green background is uncultivated, so mostly wooded), as well as elsewhere within Laimunt Valley. That is true in much of the Jura mountains.
But the areas with the strongest aura of the covenant have been cleared of wood. There's an area around the magical oak has mostly been left alone, but it is not dense enough to be called a forest.
I really mean "enough to count as forest for a Forest Path". Which I don't believe has to be the Black Forest but needs to be a forest with a significant magic aura.
Yup, any forest with a Magic Aura will do. Stronger Auras are helpful for learning.
Then the wooded area at the covenant wouldn't make the cut.
There doesn't appear to be a unified, large forest in the Jura -- it tends to be parcelled out by the mountains. So I'm not sure if a Forest Path would be possible.
However, a Mountain Path might be an alternative. HoH:MC p.103 lists Mountain as a sample Nature Lore. The Jura mountain range is large and wild enough to have a magical aura in broad areas.
The Oak of Binding Promises
The peasants who live in the villages nearby know about the oak at the entrance of the valley. They say it was the druids that planted it a long time ago. They say, the oak tree witnesses promises and rewards those who keep them. This makes it a popular spot for young lovers and for outdoor marriage vows exchanged between peasants. It is said that if you come back years after a vow you have kept, the tree rewards you with a golden acorn. This only works for vows that mean something, not for silly little promises. The covenant buys these acorns from the peasants because they contain herbam vis.
Father Luc, the local priest, isn't too happy about these pre-Christian beliefs, but since he is a kind man, he agrees to the peasants' pleas for marriage ceremonies under the old tree.
Story ideas:
This may have been different than it is today. There was an incredible amount of deforestation going on in the 12th and 13th centuries. Alpine grazing in the form of seasonal tranhumance has been a factor in the Alps vegetation since the Stone Age, but it increased in the Middle Ages (Alpine transhumance - Wikipedia). This is real world data, and so it may be a bad idea to try to use it for Mythic Europe. But if we wanted to have a forest, we could always claim that it was there.
So, I just discovered that the Jura mountains have lynxes (well, they were reintroduced after being extinct on 19th century. This paper also has a neat contour map of the mountains.
Whoever is wandering around these mountains in the future, beware!
Also, I just discovered which animal Regulus will have as a familiar. Lynxes are said to be able to see through objects and reveal hidden truths.
I like the description! 
Question to the troupe: Is everyone ok with this vis source being Herbam? That seems a bit divergent with regards to the powers ascribed to the tree. Based on the description, I would have expected it to be Rego, or maybe either Mentem or Vim, rather than plain Herbam.
I propose that the tree has a crop of regular acorns every year, that are Herbam vis, but if a suitable promise results in a golden acorn, that gives an additional pawn of Rego vis.
I second that proposal, with a twist: a broken promise is one less pawn of Herban.
It would also be nice to have an yearly festival, some kind of fair, around the oak. Maybe sponsored by the covenant. That would make it easier for:
Maybe the fair doesn't exist in 1204; maybe it existed until 20 years ago, but not anymore. Or maybe it already exists and the magi can give a little help to make it even better.
I would not advise that, because you might end up with no Herbam at all! 
A festival of some sort with the villagers attending would be interesting. Reinforces the local traditions. It may also make the local priest frown at the pagan undertones and involvement of the magi.
A fair often involves merchants and many strangers, so I doubt that would be a good thing on the covenant's doorstep!
I think it can be an old tradition, which may be what attracted Leonardus' attention to the valley in the first place.
I'm ok with a mixed Herbam and Rego source.
I've added the Oak in the covenant's vis sources, with a few changes. All the is missing is the quantity collected each year. Suggestions?
The agreed total for the covenant is 60 paws/year, right? Do we have a target number of sources? There should be at least one in each tribunal, right?
I think 5 or 6 sources (with the oak counting for 2) could be nice. 15 paws Herbam and up to 5 pawns Rego? Idk. Now that I wrote it it seems like a lot.
No target number of sources, but I was thinking a few main ones and several smaller ones. At least 10-15 pawns which are clearly in each Tribunal's territory, so that no matter what direction the covenant takes there will reason for conflict (contested vis sources).
Then I'd say 5 pawns Herbam, up to 3 Rego, but usually only 1.
How far away from the covenant does a vis source have to be so that it is reasonable discuss if it is in another Tribunal? 3 days travel? 1 week?
Anyway, I'm thinking of the following:
In spring, summer or autumn, after traveling a few days south towards the mountains through an old Roman road, a traveler might decide to exit the road towards east just after a huge broken rock with roman inscriptions, and walk until he finds a stream. Following the stream to its origin you will arrive at a cave; a very dark cave.
For only a few minutes in the end of winter the sun is in the right position to shine inside the cave, and a single ray of light reflects on a frozen pound in the cave floor, that immediately melts. That water shines with gold light and contains Ignem vis, but if not harvested in a few hours the stream awakens, flows down from the mountains, and the vis is gone.
About 26 km from Laimunt there is a pierced stone, close to Porrentruy. The coordinates are (if anyone wants to check: 47° 24′ 21.6″ N, 7° 07′ 07.28″ E
I'm thinking something faerie, but not sure about what yet.
There is also the Clendy Alignment about 100 km away (but then we are talking about around 5 or 6 days by foot to go, another 6 or 7 to return, and a lot of ups and downs through the mountains).
Anyway, interesting places. I love google maps.
I think 1 to 3 days away (for a mundane), I think. Basel is about a day away, just for reference (the city would be out of bounds because of the redcaps' strong presence). Travel is clower in the mountains, of cours, but that still leave a fairly wide area.
Using modern names, the area would be from Besançon (west) to Zurich (east) and from Mulhouse (north) to Bern (south).
Nice find on the pierced stone!
@Arthur do you have any strong ideas for the vis from the snake? Your recent description of it and the galley kinda matches something I was thinking: it somehow sheds its skin, even petrified (maybe in large flakes, like flakes of mica) and the petrified pieces of skin contain Muto vis.