A couple of observations. Firstly broadcast safe is a dud filter. Levels can be legal with this as last filter until render and then levels do not render legal. I stopped using this filter years ago and prefer RGB limit which is much more accurate post render, although not infallible.
Secondly Baz,how can a smart filter on a clip know about the other layers like supers etc which, depending on composite modes, can send a legal clip over. Putting a limit filter on a nest is the only alternative but really, the only logical way to broadcast limit is on the final SDI output. If this can be a master setting in FCP or an option on an I/O card, this to me is the only logical way to deal with this. A clip based option is never going to be satisfactory.
My preference would be for a hardware limiter as part of the I/O card.