It's probably something akin to what I had to do to get my 122 key emulator up and running at full. By default, linux disables all the non-standard codes. Or rather, more specifically, the kernel receives them, but does not pass them along to X, as, by default, X doesn't know what to do with them.
On my system, I had to run the script I posted above to have them recognized by xmodmap. I actually have a full script that runs at boot that reassigns all of the codes my keyboard generates to known X mappings. It's mostly redundant, but it does what I need it to do. Plug in your terminal board, start hitting keys, and pay attention to the system log. That's how I found the missing codes on my system. Pop a script like above into your rc.local (or whatever your system uses), and you should be good to go.
I really wish I could find the website that gave me the information I needed to get this going. I'll check on my workstation in my office, see if I bookmarked it there.