It should be possible to make these things work on BSD.
The DRI2 interface is what you need to have in order to communicate between the userspace Mesa-based drivers and in-kernel memory management stuff.
So if the BSD folks are able to modify their kernels to support the DRI2 interface and it's limited set of operations then compatibility should follow.
Probably the biggest problem is that I suppose the BSD folks rely on X drivers heavily for their GUI stuff. With the DRI2/GEM/KMS/Gallium3D stuff the need for X.org's DDX drivers are evaporating.
That is in the future the need for things like xorg-drv-video-intel (or whatever) will be gone. Instead of running your GUI as root you should be able to run it using normal everyday APIs that are available to any other application under your user account.
So unless the BSD folks get their butts into gear then they will get left behind and end up having to force to deal with a huge amount of Linux-isms... It's going to be very difficult for X.org to avoid shovelling to much Linux-specific code when the interfaces they need in BSD are simply non-existent.
One of big features for Vista, for example, was a new userspace driver model for graphics. As part of it they touted a new kernel memory manager for graphics. This was one of the big issues that early Vista users had... their video drivers from XP wouldn't work that well and the new vista-specific drivers were not mature.
And I expect that if anybody here was to look at OS X they would have a similar setup for their composited interface.
So I think that if you want to provide modern graphics and modern APIs that take advantage of GPUs this is just the sort of thing you need.
Plus moving your GUI out of being setuid root binary is always going to be a good thing.