I have seen them in reef tanks and known people on reef boards who have kept them in tanks, however I have not myself. They have always appeared to be nice reef fish. You should be good to go except for shrimps and I wouldn't try really small fish with them just to be safe. I believe they are rather shy.
Reef safe is such a broad term. A marine beta is reef safe in a sense because it won't harm corals. However, it may eat smaller fish and things like shrimp. (but who knows as I have a pair of cleaner shrimp that will pull food straight out of a large clown triggers mouth).
If you're not worried about possible losses, I don't see any reason why you can't have a betta in a reef.