The only thing that prevents you from having other combinations of multiple package memberships is that there is no UI that allows you to specify which combinations of packages a method should be part of. The category hack is great because it works with existing tools, but existing tools don't allow for methods in multiple categories. If you fix that you get your multiple package membership in Monticello for free. Alternatively provide a UI to specify which other packages the method should be part of, and you're there, too.
That would be very useful !
Stef