Thanks, probono, that totally worked! I unpacked Wireshark, then added my own custom diameter definitions and dictionary.xml file to "usr/share/wireshark/diameter". After repackaging, it worked like a charm!
However, I hated the idea of having a portable wireshark with our propriatary diameter definitions embedded inside, so I "crippled" my version by removing the "usr/share/wireshark/diameter" directory altogether, and replacing it with a link to a static location on the host filesystem. This way, Wireshark is still portable, but can't decode diameter unless that directory also exists.
It's a half-assed solution, but I didn't want to create a portable wireshark that could "fall into the wrong hands...".
I only mention it because I was surprised it worked, and maybe someone else would find it useful in some few edge-case scenarios...