In our intents _ex stands for EXtended, which means it can be used not only for this particular new scheduling policy, but for all the ones (if/when they will come! :-P) that needs the EXtended sched_param structure.
(It is the same name used by Xenomai for the same purpose, i.e., supporting new scheduling policies with extended parameters, such as POSIX's SCHED_SPORADIC.)
I think it _is_ a good thing to keep this general but, hey, we can discusss about that! :-)