What I think would make a fun crossover would be to have a case that for some reason requires Castle and Beckett to interview detectives of the 12th precinct who were active 30 years ago. So they go and talk to cast members of "Barney Miller, " including detective turned novelist Ron Harris. Harris was of course played by Ron Glass, who also played Shepard Book on Firefly.
Castle can even mention he's worked with Harris in the past.
Or is all of this too much?