I think the content interface improvements that were discussed to support desktop themes in snaps might help here.
When implemented, this will allow you to configure the content interface such that multiple slots will attach to a single plug. In this mode, each plugin snap would provide a content interface slot exposing their plugin code, and the cups snap would provide a matching plug.
On the cups side, you could pick a directory where the slots would be exposed, which would expose a tree looking something like this:
[files from first snap]
[files from second snap]
The plugin snaps could control the
snap2, etc directory names, but snapd might rename them if there are conflicts (it won’t be able to merge content from two snaps). Does this sound like something you could use?