Apart from Google (try to find FreeDOS documentation, I remember it was somewhat better), there is also the following:

http://depot.javispedro.com/nit/dosbox/kc.tar.gz -- I ported FreeDOS's "key compiler" to ANSI C, so that you can build it on your N900 or a desktop. Includes Makefile. Also includes the original documentation which describes the key file syntax.

http://depot.javispedro.com/nit/dosbox/rover/ -- Source files for the rover.sys I created. They are not in UTF-8 but rather in DOS codepage 850, so they are hard to edit unless your editor supports that. An example of a somewhat good editor supporting such code page is DosBox's edit.com

You might want to use
http://depot.javispedro.com/nit/dosbox/rover/us.key
as a starting point for key file, but I don't really remember the syntax.... I however remember that every entry in [keys] should be like this:
where scancode is the "real" keyboard scancode (suffixed with C if it's a key that can be influenced by "caps lock"). For every plane, you can set either a command or a character. Commands are prefixed with ! (e.g. !0 is "command 0" which means 'use defaults').

Planes in the us.key file are:
1. Normal
2. Caps lock
3. Shift + N900's FN
4. FN

They are defined in the [planes] section (first two planes are implicit).