Of course, a prerequisite for using foobar is that you let go of the vague concept of "now playing"
If by "grouping" tag, you're referring to the TIT1 frame in id3v2.3, then iTunes, WMP, foobar and MusicBee are some of the players I can think of off the top of my head that recognize this tag. I think the problem you'll face is not so much finding a player that recognizes this tag but one that allows you to use it as you wish (for example, WMP writes sub-genres to this frame if memory serves me correctly). I know foobar will allow you to use it however you wish, and possibly MusicBee as well.
What I'd do is find a player you like and do some research as to whether it supports the TIT1 frame and whether it has appropriated it for a particular use. My advice, however, is to just add your "grouping" info to the title tag to ensure you'll see it regardless of what player you use.
My advice, however, is to just add your "grouping" info to the title tag to ensure you'll see it regardless of what player you use.
Not sure I follow. Is foobar going to fracture my laughable human psyche and allow me to realise that all moments in sonic history can be experienced simultaneously?