As an outsource partner I run into this regularly where someone makes an asset and its not all in the same painter scene. which causes extreme confusion and a lack of ability to look at an asset as a whole outside of marmoset.
SO! I propose that being able to merge painter files, much like you can merge max scene into one file.
I mean it couldn't be insanely difficult as each mesh element has its own separate material applied, therefore would display the same way in the painter scene as it would if it was set up that way from the start.