I have released an OpenWRT buildroot patch set that constructs an image with Asterisk 1.8 LTR integrated into it. Asterisk is inherently a B2BUA by virtue of its architecture, so it seems to me that configuring it to act like an SBC is likely quite doable. You should see the last couple of pages of the Metarouter images
thread if you are interested; look for posts from me.
If you don't want to use Asterisk and instead prefer something more suitable to task, my understanding is that forum member liquidcz
has built the entire set of available third-party package "ports" for recent-ish versions of OpenWRT 'trunk' that run under MetaROUTER, which should include both OpenSIPS and OpenSER/Kamailio. Details, again, are located in the same Metarouter images
thread; look for liquidcz's posts which contain links to his most recent OpenWRT builds as well as his package repositories.
As to OpenSBC itself, it doesn't appear anybody has tried building it for/porting it to OpenWRT; at the very least it is not in the list of packages on OpenWRT's SVN repository. In fact, the project may be dead, I'm afraid...www.opensourcesip.org
is occupied by a domain squatter now, and Solegy's page
seems to be devoid of content.