That's a good question. It is in the Brassicaceae family which includes mustards, the crucifers, and the cabbage family. So it includes things like mustard greens, broccoli, kale, cabbage, turnips, horseradish, brussel sprouts, and garlic mustard (not actual garlic). All of those are safe for rabbits to eat (at least in moderation). That would make me guess that alyssum is probably safe, but I can't say for sure if it is safe.
I'd be curious to know if anyone has tried it or knows more. I'd be hesitant to allow a rabbit to eat any until its safety is verified.