Implementing it is straightforward, trivial even. As makomk hints, the problem is that you'd be doing something non-standard and thus have to rely on your own

Implementing it is straightforward, trivial even. As makomk hints, the problem is that you'd be doing something non-standard and thus have to rely on your own driver (or someone else's) rather than the system's built-in and ready-to-go hid drivers. Or you show up as multiple keyboards simultaneously, but that confuses some software.