Workflow for working with FBX submeshes / Materials / UDIMS?

Ok, so, I´m bringing this up from discord.
I´ve been trying to texture my metahuman project since Instamat came out, because that was previously only possible with substance or mari, which I didn´t have access to.
Now, the problem with Metahumans is:
The body and head mesh are separate and use separate materials in Unreal, but the textures are mapped in UDIM space.
All the other meshes that are part of a metahuman (including the eyes) are also separated with materials, but all are mapped in UV 0-1 space.

Which currently prevents me from texturing everything in ONE mesh in Instamat, if I wanna use the base textures, as I cannot mask the different parts of the mesh properly:
-Unified material doesn´t work, as the UDIMS mess up the masks.
-UDIMS don´t work for the parts of the mesh, that do not use UDIMS.

I can neither mask correctly using UV shell masking (which is a great addition otherwise) nor Mesh masking (which I think actually works in material layering, but definitely doesn´t work in UDIM layering projects).

So, not sure what to suggest here, maybe a unified material/UDIM hybrid bastard solution?

Only other workaround right now would be to create an extra mesh in 3ds max and use a script to bring the UDIM textures into 0-1 space and then use unified material workflow instead of UDIM workflow, but if I´m gonna do that more often, its gonna be cumbersome and requires me to keep track of even more meshes for different apps…

Maybe I´m overlooking something or there is a bug for UDIMS, but I´m guessing it has to do with the way you´re masking UV shells/meshes with point locations or something…

Here is the Mesh masking that looks like it should be working, but isn´t:

And here is the UV shell masking that isn´t working (red lines for emphasis, looks like the eyes are also reacognized as a separate UV shell, but its not working either.

Thanks for your post! Just to clarify, you’re looking for a way to texture meshes in a layering project that use multiple modes for their material sections? Ex: A few meshes use the UDIM workflow while others use the Multi-Material workflow, and be able to texture them all within the same layering project?