Taoman, azrobert, thanks for your speedy replies.
Taoman: Sorry my initial post was confusing. What I was trying to convey is that, yes, all four SP1 - SP4 profiles are already in use, so as azrobert points out, if I want to use XMPP, I'll need to double-up on one of the SIP channels.
azrobert's solutions are always works of art, and this one is not an exception. It's elegant and I'm sure it would work. However, I rather like Taoman's last suggestion to go with Simonics. If I did this, I think I'd drop CC entirely and use the Simonics gateway bidirectionally. The reasons this looks preferable to me are (1) it's more portable (I can pick up the OBi and take it with me to a new location without having to reprogram anything), (2) both Simonics and CC provide CNAM, which is the reason I was using CC in the first place (although they sometimes yield different results and I'm not sure which one is better), (3) with Simonics, I can keep each trunk (both the incoming and outgoing functions) on its own SP profile which is less complicated, and (4) with Simonics, you aren't tied down to the obitalk portal for programming. The main downside is that annoying moment of silence you have to remember to account for when you answer an incoming call.
Anyway, you two have given me something to think about. Thanks again to you both!