Just reading about OpenStack, in particular about supported hypervisors. Is there any technical reason why bhyve isn't on that list or is it just a matter of it being too young, immature or a lack of time/interest from the community?
Yes part of the base system isn't production ready... bhyve is in base so people can use and test it, and for some guests it works very well, but it's far from being tested enough for anyone to seriously rely on it. (Having said that people do run it in production and while I don't, I have FreeBSD & Windows test guests that seem to run perfectly fine)
There's nothing stopping bhyve being supported by OpenStack, although someone obviously needs to do the integration work. It looks like OpenStack can use libvirt, which I believe already has support for bhyve, so some functionality may already be possible. Of course bhyve is new and extremely limited at the moment, to the point where it only really makes sense for test/dev.