It certainly would seem like merging build farms or continuous integration systems for the ports should be doable. The setup for such a thing wouldn't be free, but it would benefit all the BSDs if they could agree to it, and make this business of supporting esoteric hardware a little less of a sticking point.
The thing is they had their schisms for a reason, and inability to work together is kind of baked in, even if much of the code does in practice cross pollinate further downstream.
The other question is if it's possible to get the benefits of the strange old hardware support without the old school power consumption. I think many relative youngsters would be surprised by how thirsty some of those classic UNIX boxes were, and the hardware must be close to dead now. At some point the only remaining working SPARCstation will be used to build OpenBSD.