So what about Java Applets? There should be Java support for almost every platform. But then we're getting damn close to the jMax way of doing it... and they seem to try to get rid of their Java GUI for some reason.
I've been trying to communicate from proce55ing lately ( www.proce55ing.net ) it's very easy. comunicating with pdreceive is trivial with the builtin net functions (just adding a trailing ";") but you can also use OSC via javaOSC
I think it's the perfect platform for applet based GUIs
cheers,
gerard