Hi MikrySoft.,
Please review
www.usb.org/developers/devclass_docs/Hut1_12.pdf (e.g. page 53).
You'll see that there so much of possible HID codes, that an online Rockbox GUI interface to allow custom mapping is not reasonable, and will result in noticeable bin size increase if implemented.
A different approach is to use keymap files which Rockbox will load automatically. This will require some sort of external editor to edit these files, either plain text editor or even a custom-made editor.
If you desperately needs different keymapping you can edit the code yourself and compile.
I guess customization is always good, but in that case I would like to hear why you think it worth the implementation effort, and possible binary size increase. How do you think a large number of users will benefit from this?
BTW the USB HID is a sort of 'side-kick' in Rockbox, so don't expect so much enthusiasm by the developers to further expand this feature.