Realistically we need to get the common buttons, menus, lists, scrollers, choosers and other gui elements under our umbrella. Support for more exotic classes (and for MUI this is quite a lot, I guess) is of lesser importance to the target audience.
Sorry, but I have to disagree here ;-)
The 3rd-party-classes are one of the biggest arguments for MUI (and sometimes the
biggest against it :-o ).
Coding a "real" MUI-app without useing those, is next to impossible, unless
you create your own classes inside your project, which would most
likely break the compabilty with your ReAction-layer.
MUI and ReAction are just to far apart for your idea to turn into reality(without
adding lots of bloat).
Better solutions:
a) use MUI/Zune which is (will be) available for all 3 plattforms.
b) write some headers that make porting between MUI and ReAction easier (but
would still result in different exes).
c) create a new GUI-lib useing only the 3.1 API (like that NewGUI,Triton, StormWizard,
BGUI .....), and make it open-source.
