I'm pretty sure the reason Ultimarc (and several other companies for that matter) don't make what your looking for because Microsoft has locked everyone out.

Theoretically, you might be able to buy one of Ultimarc's PS3 adapters and hook it up to one of the PS3->X360 adapters out there, but even if that did work, the weak link in the chain is going to be the PS3->X360 adapter. You're probably going to get lots of lag. Again, that's assuming that setup even worked.
Your best (and most cost-effective) bet is to get some hacked X360 controllers and interface them to your I-Pac.
I just recently looked into the same thing -- I wanted to hook the mini-pacs in my 4 player panel to my 360 -- but I'm no good with all the soldering/hacking. So I'm probably going to stick a Playstation 3 in the pedestal and get a couple of Ultimarc's PS3 adapters. That way, I can still play Street Fighter 4 and Castle Crashers which is coming out for the PS3 next year.