There's already a few ways to do this in MB for single tags & custom groups... not for multi or cascading/relational facets though unfortunately...
Maybe this helps - the way I use this is through the Thumbnail Browser I set up to achieve the filtering part of this...
i.e. settings - always show - no thumbnails - use top filter *Name*, i.e. Genre Category (eg "Electronic" - achieved via the current Genre/SubGenre definitions inside *\MusicBee\Configuration.xml ), Genre (eg "Experimental"), etc, or custom ones.
Eg, this is the thumbnail browser, switched over to show all 'Instruments' custom tags I have in my library, which gives me a list of all of them, which in turn acts as a quick filter, and through that I can easily filter the main columns to only show/play all tracks I have tagged with '808'.
c.