I don't know if this interests you, but I am considering this for my much-delayed HTPC project.
I already have the kit minus the RS232 cable, but the remote control distribution logic is not as straightforward as I originally thought, so I might have to rethink my mad world domination plan. Here's a link to a thread where Gonk, Elmosaurus and yours truly got into the subject of IR distribution on the 970 and 990.

I only bring this up because it sounds like you might want to send commands to the 990 via RS232. I would expect that to work. Now passing those commands to the IR-out port on the back of the 990 is not mentioned in the manual so that part *may* not work. If it does, please let me know.
One thing we do need is the RS-232 commands for the 990. If you read the "Outlaw guide to the SMS-1", Appendix A, you'll see what I mean.
The outlaws did a nice job with that SMS-1 manual. It would be nice if some outlaw out there did something similar for the 990. Although I never seem to have enough time to tinker...