DragonFly BSD was forked from FreeBSD because of fundamental design disagreements between Matthew Dillon and the rest of the FreeBSD developers. Thus I doubt that the characterization as "research platform for FreeBSD" fits. You might as well say OpenBSD is a research platform for NetBSD.
But that does not make Dragonfly a "research platform" for FreeBSD. Both systems have different priorities and goals.
(Although I wonder if any of the things that Dragonfly set out to do differently has been adopted back into FreeBSD?)
Don't forget it was originally FreeBSD 4, one of the most stable Unix-like operating systems of its time.
The last time somebody tried porting HAMMERFS it was discovered that the systems are already divergent enough that it would be a huge job because to stay true to form, other Dragonfly's systems upon which HAMMERFS is built would also need to be ported, and as such systems include such basic stuff as the threading model, the effort was abandoned.