I've seen some sneak thru, but I think it was always after combat ended (mainly on a wipe when some other crappy bossmod didn't disable). Does BW's built-in Bossblock enable/disable all the time, or is it always on?
Okey, other question! What add is the source spammer using? Not to be suspicious, but some crafty bossmod dev might be sending their messages with a non-printing char inside the *'s so that it doesn't get blocked. Not that I think anyone would think their mod is so important they have to override mods installed on other people's computers...
You should only be hooking RaidNotice_AddMessage if you intend to change it's behaviour (block crap). If you're putting the messages somewhere else, you should use the event that drives the RW messages to drive your output.
So, hook to block the RW message, and event to output it somewhere else.