Well, anything is possible in principle and that's what the Requests forum is for.
I guess in your example, 'Mandatory' would be the equivalent to a tweak to the Deny options, to deny when a string isn't present, say with a ! switch, !*@channel* | D.
There are indeed limitations to a filter system that only applies to subject lines and can't, for example, make use of other fields (e.g. Description often contains valuable info for potential filtering).
BUT increasingly I think there's an expectation that sites will handle most of the heavy lifting through their RSS or API options. In general, I would say that Alt.Binz filters work best when they're applied as a final 'cleanup' stage for highly customised RSS urls. This is obviously much easier now that we have multiple RSS feed support in Alt.Binz.
So in your example, I would look for and use a site that allowed me to add a search filter to the RSS url, and create a new RSS entry for Alt.Binz (e.g. blah_rss.php?search=@channel).
So basically aim for more RSS urls with shorter filter lists rather than trying to squeeze everything into one. There will be a range of views here about that.
TL;DR - The best filter to aim for is this one:
* | A