I've been contacted by a company who will support further development of OpenBSC, including the integration of support for the ip.access nanoBTS. Those beasts are unlike any traditional BTS, as they don't use a E1 link but rather route everything over TCP/IP. They even use Power-over-Ethernet and look almost as innocent as a WiFi access point.
After yesterdays meetings were finished at 7pm, I spent another eight hours of intense hacking and looking at protocol traces between a commercial BSC and the nanoBTS. It seems like everything is as far as possible according to the relevant GSM specs 08.58 and 12.21, which means adapting OpenBSC will not at all be hard, at least not for the signalling part. I haven't yet looked at the actual voice channels, but I'm confident it will all work out just fine :)
It turns out the abstraction of the input interface and introduction of something like a "BTS link driver" during the last weeks has helped a lot to ease the integration of a completely different BTS with a TCP/IP based link.
Unfortunately I currently have to deal with some rather dull and boring other work, but hopefully will be able to continue a lot over the weekend.