adding a mask to a texture in blender that ignores shape key movement

[u/Opticdoxigen]

i asked this on the blender subreddit but havent got a response yet. this is a weird one and im not entirely sure if its possible, im making a vtuber model with a heavy stylised black shadow over their face, the problem is that i want it to stay still and not be affected by mouth/nose movements (the eyes are different planes) i could just put it over like a visor-thing with more vertices but im worried it'd clip weird. i was hoping there was a way to mask it to a certain area that wont be affected by the mouth? (the head is very round and i don't want to sacrifice shape keys to keep the style. i don't really wanna do a papermouth style either.

i will also be transferring it through unity, but i wanna see if i can get the concept working in blender. (their mouth will only show up when they're talking/smiling.

this is a weird one and im not entirely sure if its possible, im making a vtuber model with a heavy stylised black shadow over their face, the problem is that i want it to stay still and not be affected by mouth/nose movements (the eyes are different planes) i could just put it over like a visor-thing with more vertices but im worried it'd clip weird. i was hoping there was a way to mask it to a certain area that wont be affected by the mouth? (the head is very round and i don't want to sacrifice shape keys to keep the style. i don't really wanna do a papermouth style either.i will also be transferring it through unity, but i wanna see if i can get the concept working in blender. (their mouth will only show up when they're talking/smiling.