I like svuorela's suggestion as well, if I got it right. You're saying that:
- mbarcode depends on mbarcode-all-plugins or mbarcode-plugin
- mbarcode-all-plugins depends on the core plugins
- mbarcode-plugin is a virtual debian package which is named as Provides: mbarcode-plugin in all the other packages?

I'll try to set this up now and revert the changes in SVN.