Some changes might be needed to optimize the layout (default to fullscreen windows, merged menubar/toolbars (we need a standard toolbar class anyway)), and even more if you intend to target devices without touchscreen who use a joystick-like HID, but I think primary targets would be PDA and PDA-like smartphones.
An app designed for a desktop should not be used on a PDA, not only for the different size of the screen. Probably it will be more wise to ditch away the actual UI libraries altogether to make more specialized versions.
While it's not a standard, last year I spawned the libwalter project @ OSDrawer to gather together BeOS code that was floating around the WWW which would make a good starting point for classes for R2. MIT license, OT style guidelines, and a jam-based build. A toolbar class is part of the project. I had envisioned someone examining the code for R2, but while I'm not saying any of it should be part of R1, I wouldn't complain about it, either. :)
Well, that part about the OT style requires some work :-) Regards, Gabriele