-cc list trimmed to those who have recently responded.
Here is a patch to go on top of 2.6.11-rc2-mm1 that fixes some bugs in the general SCHED_ISO code, fixes the priority support between ISO threads, and implements SCHED_ISO_RR and SCHED_ISO_FIFO as separate policies. Note the bugfixes and cleanups mean the codepaths in this are leaner than the original ISO2 implementation despite the extra features.
This works safely and effectively on UP (but not tested on SMP yet) so Jack if/when you get a chance I'd love to see more benchmarks from you on this one. It seems on my machine the earlier ISO2 implementation without priority nor FIFO was enough for good results, but not on yours, which makes your testcase a more discriminating one.