I guess I misunderstood - you said "as a developer" which I read as you wanting to access the HAI serial protocol to find out what it's really sending.
The question seems to be if the HAI OmniStat is sending humidity and the Elk isn't receiving it, or does the HAI not support sending it over serial to begin with?
If the Elk doesn't receive it, that's in the XSP or other Elk components - you won't be fixing that... only Elk can. If HAI doesn't send it, well - you're not going to have any luck getting that either!
If it were me, I'd find out how to talk to the RC2000 as a PC - in that case you'd want HAI resources to find out if it's even capable of sending humidity; if not, you're barking up the wrong tree. If you confirm it can, then ask the question of Elk when they might be able to add support.