More digging - updating progress here for the internet archaeologists of the future. I’ve updated to and tested 4.20.2 but the behaviour persists.
I have had a small breakthrough though. If I package the plugin via the editor, then in the packaged output files, remove the Source*\Private folders and in the .Build.cs files for modules used in the plugin, add “bUsePrecompiled = true;”, the C++ host project will at least build successfully with the stripped back plugin (as it seems to skip compilation for the plugin entirely). However, it fails to compile DebugGame or DebugGame Editor, and if I rebuild or clean the build configuration it will delete the plugin binaries/intermediate files, forcing the user to manually copy them back in place again.
So that’s some progress at least, but I’d really love the opinion of someone at Epic on this, as I think the bUsePrecompiled functionality is mostly used by the engine to distribute binary-only versions of modules through the launcher. It makes sense they wouldn’t include precompiled versions of DebugGame, since this functionality is only intended for engine-side code, not game-side code. I could be wrong though.