Can fighters from all styles (stand-up, that is) join the Sabaki Challenge?
I found on one of their websites that you must be affiliated with NIKO (New International Karate Organzation) to receive an invitation. But I know I've heard of other fighters competing it. For example, back in the early nineties, they gave honorable mention to a praying mantis fighter (I think??) who was disqualified because of an illegal technique (I think a spinning backfist), but who had epitomized the warrior spirit.
Fighting Spirit is 90% of the battle. If you don't have it, you ain't **** -- regardless of what style you practice.
Sabaki challenge is only for Sabaki student. It is almost identical to Kyokushin style fighting but Kyokushin is open to all styles.