Judging from the installation instructions, it seems preemption is there:

cd litmus-rt
patch -p1 < ../litmus-rt-2011.1.patch
# create a working kernel configuration
#  - select HZ=1000
#  - enable in-kernel preemptions
#  - disable NO_HZ
#  - don't use power management options like frequency scaling

#  - disable support for group scheduling
#  - disable "Write protect kernel read-only data structures" (in kernel debug)

I got the feeling from reading that the main thing missing is some kind of sane interface for assigning RT priviliges to non-root users. Then again, that maybe covered to some degree in the provided liblitmus which in turn provides:

rt_launch, which can be used to start arbitrary programs as real-time tasks.

If I get adventurous I may try compiling this thing to see what I can see. Most of it is over my head and I'll take your word for it that this is far from userland friendly or ready, but it looks like a fun learning exercise.

-m