This sounds like an interesting project. Have you included it in the Poly/ML software directory at https://www.polyml.org/software/index.php ?
I just did: https://polyml.org/software/details.php?id=46.
It might be possible to fix the opaque type problem but I'm not sure how useful it would be. I get the impression that most users of Poly/ML build their own executables from source and so would need to compile the modules they need anyway.
My intention was to enable recompilation speed-up during a development cycle by caching modules locally to the project, rather than a system wide cache. Similar to PolyML.make but with on disk persistence so that one would not need to keep the building process running at all times.