This would be really useful for me, maybe as an option in an existing UI add-on like Foo Ui Columns.

Something like this-
Mockup

I don't know how hard this would be to implement, but I guess it wouldn't be too complicated. Anyone else think this would be a good addition?

The missing link!

I sure hope we'll see something like this soon. If I'm not mistaken, musicmusic is planning to add support for sidebar panels in UI Columns, and this is IMHO a good candidate for a panel.

I would love to have that too. I found this program for some days ago, called Musik: http://www.musikcube.com/html/cube_screenshots.html

If you look ath the top you see artists and albums in listboxes.
I really like that function. You can choose the artist you want and you get all the songs by that artist, and you can go back to the playlist you were before.

Hope it will come to either foobar core or Columns UI.

Yeah, musicmusic said he will allow possiblity for 3rd party extensions to control the playlist sidebar in his columns-based UI. I hope it will be available with the upcoming 0.1.1 final release.