House Bjornaer, Clan Arelie, Oath of the Empty Hand - You vow never to raise a weapon against another being. Does this include claws and other natural weapons?
This seems a little bit too much like "letter of the law over spirit." The intention, by my reading, is simply to say that those who take the Oath will not retaliate to any living being with violence or threat of violence.
Though I guess even if you take the "letter of the law" approach, even fists are on the Melee Weapons table... 
This was a reasonably common issue in the middle ages, where clergy were often sworn not to draw blood, so they smote enemies with a mace instead. This quibble was so prevalent it actually defined "clergy" in That Other Game.
I'd say decide for yourself, but use that same logic elsewhere in your games, i.e. if you have fighting bishops on the battlefield, then avante gard, Bjornaer!
Note that if you're going to quibble, you can still use siege weapons, magic to hold enemies in place, and other such fun and funky things as a trained team of psychopaths... sorry, grogs...
Hey, did you hear about the maga who took a vow of silence and had a Line-range Creo Mentem rote? Me neither...
Anyway, if you're the sort of individual to quibble about the letter of a vow, I wouldn't stop you from pulling that kind of twist. It is in-theme to have a very narrow view of what "weapon" means, though obviously your clanmates may disagree with you.
Is there actually more than one documented instance of this?
If they were that literal about drawing blood was their meat always cooked well done?
Smashing someone over the head with a mace is actually going to draw a great deal of blood.
depends on the helmet they are wearing. Also the head is not the only possible target- a good mace blow to the abdomen can kill a person without directly drawing any blood (though they are likely to vomit quite a bit of it)
This is where the quibble comes in - yes a mace draws a LOT of blood, but it's not INTENDED to do so, like a sword or a pick. And with religion, it's the intent that counts.
As for citations, I know it's mentioned in La Chanson de Roland, and I recall it from from a lot of crusade stuff, but I don't have any other references offhand.
And do anyone have any references to anyone besides that specific individual?
Um, the Hastings tapestry?
If you are trying to debunk the whole no spill blood thing, I think that job's been pretty well accomplished, but this is Mythic Europe, where the legends are real.
Do two references and a lot of conjecture actually prove anything? Heck, no! But it's too good a story NOT to use.
I would say that it depends on the situation you find yourself in AND what the SG rules.
If the restriction is chosen by the player, then that character should enact it as part of their in-character play. Otherwise it's just backstory fluff at best.
As SG I'd implement some story consequences of breaking the oath. Doubly so if the player never had intend to follow it.
Another potential is to have the character designed to break the oath, well that's fine then.
To my mind, the issue isn't whether claws are or are not weapons, but is instead whether the person who swore the vow is trying to weasel out of the spirit of it by playing lawyer games with the wording.
Vows are something of mystical significance; usurping the core tenets of the vow by playing lettering games would, to me, mean the mystical things the vow provides would be in danger of unravelling.
For those that do want to weasel out of their vows, there are absolutely means of getting the same end result with rules-lawyering entirely expected. It just isn't granted by the magic realm....
So if a player was trying to find a way out, I'd present them with an 'alternative' ancestor spirit that would be happy for them to quibble over whether claws are weapons. I'd probably also have that ancestor spirit advising for a whole lot of raising not-weapons at every opportunity...
While I agree that rules-lawyering is more of a Faerie thing, I don't think that the Magic Realm is immune to it. The exact words of an oath have power too.
That said, I do agree that the ancestor spirits of Arelie (or, since they don't exist, Faeries who imitate them) will likely be unhappy about such rules-lawyering. Plagued by Supernatural Entity is not inappropriate.
If someone wants to essentially lie to their ancestor spirit for power, I wasn't exactly thinking of the Faerie realm as the alternate source of such power.
But that works too.
There's another realm that's even more at home to rules-lawyering. Practically the advocate for it, even.
I will say this- to me claws are not weapons in this sense- your hand is still empty. If I grew out my nails and scratched someone it would be no different except in degree.
Well, yes, there's another realm that loves rules-lawyers, especially with a light wine sauce.
I just mentioned Faerie because rules-lawyering is quite literally the essence of Faerie. Faerie creatures are made up of rules, and usually are required to adhere to the strict letter of oaths. Also, because a faerie really is more likely to take on the role of an ancestor spirit and try to punish this kind of player than a demon is likely to try to tempt them. What they're doing is not a sin any more than using a mace to avoid irregularity is a sin, in my opinion.