|I think Trackback on threads would be a great idea - not just internally either, I sometimes link to Barbelith threads - and it could be done I reckon, but it might kill the poor old server if people used it extensively, which I think would happen if it was automatic.|
I don't know if there are any sites that use that sort of system internally around. Actually, Barbelith might be an excellent arena to test such a thing; it's busy but not too busy, and there are people here who would be interested in testing its uses and observing its effects.
Probably an internal non-standard pinging system would be a lot lighter on the database than an external one. Each post could be scanned for URLs internal to Barbelith, then a little bit of metadata with the URL of the referring post could be added to the post or thread being linked to. Proper Trackback would be a bit of a pain to implement I think, from what I know of the technical details, which is not admittedly all that much.