Very nice, five stars from me!
I noticed you never uploaded sourcecode to GitHub, which is concerning I do have a suggestion and could even take a crack at it myself if you didn't have time.
It would be fantastic is if it had an accompanying standalone/stub EXE - this way it could be set as an emulator EXE which would:
Launch via Steam proxy when you use the default game launch (e.g. double click)
Allow for variants to be used with it too
You would have to pass additional commandline arguments to it, though I hope this is reasonable. It could also make it compatible with other front-ends or anything really; much more universal!
Maybe the Launchbox-specific plugin could be it's own "shim" dll, and the EXE as a similar small entrypoint - the bulk of the "plugin" (i.e. the non-specific launchbox stuff - Steam interfacing and launching and whatnot) could be another linked DLL?