Mask by layer in other GP Object

I've had to separate some layers into different objects for effects and animations etc. Unfortunately, now I can't use masking between them and I really need to.

My idea of where to put the option in the current implementation of the masking ui would be to have an "External Layer" option in the Add Mask dropdown which would open another dialogue that would let you select an object, and then a layer within that object to use as the mask.

I understand it might be pretty difficult to do, depending on how rendering works with GP objects, but I feel like it would be very useful to a lot of people.