Ok, so, after looking at the last stream, that introduced mesh polygon masking and UV shell masking, I figured it might be nice, if you could pick a custom mesh for all these volume based masking options.
Justification
While I get the reasoning behind the idea to NOT use UV based masks (for greater pipeline usability) and I think you actually found a nice way to somewhat still stay procedural even with UV shell masking, I think its in some types of projects just necessary to have finer control over the masking process.
For example: if you need a specific shape, even just a random noise sphere, to mask out an area and still wanna keep your approach somewhat procedural, the basic sphere and box mesh selection options may not suffice.
Implementation Details
You could simply provide an aditional slot, where you could pick a mesh from the package for masking, instead of using the built in sphere/box meshes.
using the canvas, you could even use a mesh paint node, extract a height mesh from that and then use THAT as a mask object…