I suggested something like that to management a few months ago, a message content/keyword-filtering ability that works in conjunction with the group (or specific user) moderation setting: it would be checked & applied first, i.e. check for the key phrase/word/etc and if it found it it would do whatever the action was, if not, then the regular group-then-user moderation settings would then be checked & applied. This would allow that functionality to be used for other situations as well, not just yours; for example, the corollary to your case, in a moderated group, certain messages could be set to pass unmoderated if they contain the keyword(s).
You are wright, at the very least, your simpler wish (moderate a undesirable message even if group is unmoderated) can be very useful; right now it's either 100% black or 100% white in that functionality area, a bit of grey could help enhance its power & flexibility.
Just musing, but if a full-fledged content-filtering solution was to be developed, there's also the option of it being a paid upgrade group feature. (oh-oh, all of a sudden I hear the sound of cyberclaws on a file...)