Comment 19 for bug 313514

(In reply to comment #18)
> 1) evdev driver will query kernel for all available keys on all configured
> keyboards, then it will create table of false one-byte keysyms. In
> correspondence to them it will also write temporary xkb keycodes/evdev table.

That only works if you have less than 255 possible keys.

> 2) X will accept keycodes > 255 internally, but in protocol level it will
> provide correct keysym with possibly incorrect keycode.
>
> For both ugly hack is valid following limitation: Most applications read only
> keysym, the rest will break.

The keysym is not sent with the event. Only the keycode is sent, and the applications keep a keycode/keysym mapping table. Having two mappings and switching between them will almost certainly break grabs, as well.