Their implementation of all things Jingle came in advance of any specification, so it's not fair to call it NIH. See this comment for more info: http://senko.net/gmail-videochat-the-good-the-bad-and-the-ug...
Google needed a robust and open IM protocol, which SIP wasn't in 2004/5. So they picked Jabber/XMPP and then bolted media signaling onto it (which is patterned after SIP).
They should really support SIP by now. But it's pretty clear at this point, that they are not a player in the telco space.
Since you seem to know what you're talking about, unlike myself : D
Google chose XMPP and developed Jingle as an extension to allow for voice/video.
You could just build your setup on Google's services and take advantage of Google Apps for your domain.
Guess I'll have to check them both out more. I'd actually started a bit with XMPP but honestly I really don't like it, so was kind of hoping SIP could fit the bill .. more research needed : D
Depending on what you plan to do, if you want to be 'standards compliant', I'd look into SIP, and it's IM extension called SIMPLE.
What's interesting here is, that the 3GPP body - defining the telco standards of the future - has chosen SIP as it's signalling protocol for media sessions (voice + video), but also for IM. In addition there is a 'network address book' (= server side contact list) standard based on XCAP.
All of the above are part of what's called IMS, the IP based communications infrastructure for future telco networks.
A good library for SIP client development is PJSIP.
If you are just in need of a good IM infrastructure, XMPP/Jabber seems to be better suited.