I have had a little more time, and have been wasting it pursuing my bottom up approach...
Having satisfied myself I couldn't tlak to the RFXtrx via straight ssh commands, I moved onto Lua.
Good news is I've satisfied myself I can't talk to it in Lua either

Serial port comms in reference Lua are handled as files, no problem setting up the RFXtrx as a file and opening it for read and write, however once the reset and status commands were sent and I tried to read the response it just hangs. I suspect serial settings are the issue again.
Plenty of third party serial comms libraries, some of them fairly popular but I didn't want to start messing with things that aren't on the box by default.
This leaves me up at the level I should have started at - Luup

I started looking through some other plugins, not really got my head around the structure yet - I could say your approach makes sense lolodomo but frankly that wouldn't be much of an endorsement at this stage. I'm away tonight, however next time I get a chance I'm going to try and write myself a hard coded on / off button for some switches I can see from my PC. Once I've actually made the thing signal something from the Vera I'll be considerably less frustrated. Certainly don't hold back on doing anything on my account, if I do anything I'll post it here.
I'm running a Vera 2 - will check firmware later, however the Micasa guys reinstalled it remotely themselves because my ZWave dongle wasn't working. I'm on UI4.