I just downloaded a sample of the July issue of the current Firefly comic and the copyright page only has Fox and Boom! Studios, the publisher. So it looks like the rights for the TV series are only with Fox/Disney.
That would have been amazing.Nathan had a few Firefly cast members guesting on Castle over the years. But he never had the one that would have made the most sense: Ron Glass, reviving his role as Det. Harris from Barney Miller. Both shows took place in the fictional 12th precinct (used on TV because there IS no real 12th). Both Harris and Castle were authors, so there was a reason for them to know each other. The plot could have concerned an old cold case from the 70s coming up again, and Castle bringing Harris out of retirement to help. I'd have kvelled.