Regardless of whether you create your mental lab with momentary ritual or with an enchanted device effect, it behooves you to get your familiar in there to act as a lab assistant or just to hang out.
A simple way to accomplish this is by enchanting your familiar bond with a Rego Mentem effect to move his or her consciousness into the lab. This seems well within the capabilities of Hermetic magic even if there isn't a clear guideline. I think that base level 4: control a target's mental state is, if not a perfect fit, a reasonably equivalent difficulty. (Or would you prefer base level 5, equivalent to controlling a disembodied spirit)
Entrance of the Honored Guest
ReMe 25 unlimited uses per day
R: Touch D: Conc. (Item Maintains) T: Ind.
This effect allows one of the bonded beings (chosen at the time of enchantment) to project him or herself into the mental laboratory of the other. (insert casting sigil flavor here).
Base 4, +1 Touch, +1 concentration, +5 levels item maintains concentration, +10 levels unlimited uses per day
But I think it would be cooler to just build a hallway or path connecting the mental laboratory of the magus to the mental laboratory of the familiar.
No, the question is why wouldn't the familiar have their own mental laboratory?.
I suppose that you could just connect to the magus/maga's lab to the familiar's memory palace instead, Please don't try and tell me that the sort of familiar who would choose to bind with a maga that messes about with mental laboratories is somehow not the sort of personality that is going to pick up a level or four in the art of memory ASAP.
A mental link between minds is not sufficiently natural that it could be made with a momentary creo ritual. Because I'd like it to be a significant step up from effect detailed above, I'll make it as a constant duration enchantment. I'll adapt the CrMe 5 guideline used to alter memory palaces.
Passageway of the Silver Chord
R: Touch. D: Sun (Constant), T: Group.
A connection is made between mental a construction, such as a memory palace or a mental laboratory, in the mind of the familiar and a mental construction in the mind of the magus/maga. This appears to the bonded creatures as a bridge, pathway, hallway or some similar location that leads from the constructions in their mind to the constructions in the mind of the other. One of the two (or some other entity that has entered one of their minds) may pass through the length of the passage in a single turn regardless of its subjective length.
(Base 5 , +1 touch, +2 sun, +2 group, +1 rego requisite, +3 levels environmental trigger, +1 level 2 uses per day.)
Group because it needs to target both minds. The rego requisite is because it needs to connect the minds.
You might think that allowing your familiar access to your lab is good, but you've made your mental lab connecting to your memory palace for convenience and now you've just let your familiar have access just by opening a door and walking through. True, but remember that they've got the true friend virtue and a silver cord to protect you both.