I think the problem here is that neither XNA nor DirectX works for Linux, though, so you'd have to start using OpenGL if you wanted it to truly be cross-platform, in which case, I don't know how that would work with full-trust in browsers.
Hmm... perhaps a .SGDK2 file should be made that could be imported to support each display setup. Once each one is supported, that is. Would that be possible?