I know how much work this would be if it were to ever be implemented but when it comes to exporters Unity2D already has the backend all figured out.
I know that making an exporter that would simply make a Unity2D file is possible, although it wouldn't be HTML 5 any more. Perhpas having the editor make HTML5 games and for mobile / desktop devices we can use Unity2D.
That way, exporting would become almost a non issue for most devs as Unity2D can export almost anyhwere.