@wuzelwazel So sorry for not replying back to you. I just had a look for it now (February last year), wow almost a year has gone by. Glad to see your still working on it. I will see if I can find some time soon to have another look as well.
As far as the UDIM work goes I did something similar many years back. I extracted every texture for every triangle and wrote them to a new texture map. And as each triangle texture is extracted to the texture map I use its location for the newly created UVs. I used this for an OpenGL preview as well as rendering. Note that this is not baking at all. Since I am using actual ptex pixel image size directly for each triangle extracted to the texture. So the UV coordinates are based on the actual pixel size in the ptex images. As soon as one texture was full I created another with a new selection set for the triangles and its own material, which could be considered UDIM if I just went out of the 0-1 range, but there was not any need to for my test.
I had planned on doing this again and converting the OBJ files to C4D files in the process, to make them smaller and faster to load. Which is what you are doing with Redshift proxies I believe?
I will reply to your email as well so you have my contact details.