I have been working on this device for a while, and have come up with 
the following:

The protocol is simple:

?? ?? 52 70 ?? ?? ??
??
KC 1 [DOWN]
?? ?? 52 70 ?? ?? ??

KC	Key
48	1
49	2
50	3
51	4
52	5
53	6
54	7
55	8
56	9
57	Mode A
58	Mode B

The ?? bits represent seemingly random numbers that appear through 
/dev/input/event3, which gets created when the keyboard is plugged in.

I'm in the process of creating a userland application that reads from 
/dev/input/event3 and acts accordingly, based on user preferences.

Should it be the job of the kernel to do the command mapping (and 
subsequently create a virtual keyboard), or should it stay as a 
userland-only program?

I would be intrigued to know.

Chris