I had a bit of a play with Mono the other day and the implementation of WinForms seems very glitchy - it was quite hard to get anything to run at all, and I had to remove a load of controls in order to do so.
I think a better approach will probably be to rewrite the front end for Mono. I don't think this should be too enormous a pain, as I've tried to keep the UI code totally separate throughout - we've already got two IPlayer implementations, the desktop Player and WebPlayer. So adding a third, say MacPlayer using MonoMac, shouldn't be too hard and it will give us a nice native UI on Mac.
I guess for Linux that Gtk# would the way forward?
If we want a cross-platform Editor that will be a bigger amount of work as there's more UI there, but again the way I'm writing it I'm trying to keep the UI component as small as possible, with the "real" work happening in the EditorController component which can be used as the basis for multiple Editor UIs in the same way.
I think for Quest 5.0, the IASL and IPlayer interfaces are now reasonably stable and unlikely to change significantly, so now would be a great time to start looking at a Mac version of the Player. Any volunteers?