That can be a tricky one, and I often have to come up with a solution on a case by case basis.
In some situations, I'll make the hand Switch layer drawings to include the entire lower arm (including the glove) instead of just the hand. This solves a lot of problems...but it's probably not practical when you have a lot of hand views/versions.
Another trick is to make a Reference layer of the Hand Switch Layer (assuming you're using Switch Layer; if not, then whichever group the hand is in,)and place the reference layer above the glove. Inside this reference, you can hide all layers for that hand but the thumb. This will leave an unobstructed thumb. If you want, you can hide the thumb in the original layer, but this may not be necessary. (If you leave it on, the thumb may render just a touch heavier in the edges.)
I would then clearly rename the reference something obvious to keep you from editing. For example, if it was called 'handSwitch', change it to 'REF_handSwitch'. Coloring the layer red or another eye catching color also helps. I would keep this switch layer collapsed.
Now when you switch the original layer, the reference layer will switch along with it, and if you need to edit the shape of the thumb in the original, the reference thumb should also update.
If it bothers you that you have a bunch of layers you will never see, here's my advice: Keep that switch layer closed and just forget about it.
There are other ways but I think the above is the least fiddly one and easiest to update in the future.
Alternatively, you might consider using a 'Patchoid' layer. This is like a Patch layer but it's one you construct yourself and it can take the form of any shape. I'd have to think about how this will work and it might not even be appropriate here, so for now it's just a thought. A search in these forums for 'Patchoid' should turn up information about it.
The most
direct technique is to modify the 'sleeve' art so it reveals the thumb art. Like so...
Sometimes I'll add a tiny bone to the middle point so I can move this point around as needed in the bone layer. This keeps it easier to manage the rig keyframes. Alternatively, I might just use the magnet tool as needed or set up a smart bone control to move/reshape it...that's not usually necessary, and it may just overcomplicate the rig.
I hope these ideas get you started. I can think of other tricks but, as mentioned, tricks like these are mostly situational.
Good luck!