BSD use in embedded systems is probably as wide spread as any other OS, it just has virtually no advertising.. quite typically it gets customized by a company and maintained internally.
I'm not sure what exactly you're asking, or how you propose to modify FreeBSD, but choosing BSD as the basis for a project/product is no doubt a solid choice.. assuming you have people willing to put in the time and effort.
I don't know about turning FreeBSD into a RTOS, or what RT features you're interested in, there exists a
mailing list.. but it appears to be low volume, someone more experienced with FreeBSD might be able to answer.
There is a modified commercial OpenBSD product that I've read about called
RTMX but I don't know much about it either.