Comment 27 for bug 313514

(In reply to comment #25)
> What are the possibilities of using a synthetic keyboard for this? This
> keyboard could just remove the most significant bit, and map the right
> keycodes.

Unfortunately no, because this would break apps anyway, as no-one does the right thing of recalculating their keysym -> keycode tables for grabs when the map changes. So might as well just be Xi 2/3 and XKB 2.