Very good news;
The (in my opinion) by far best music manager/player for Windows, MusicBee, now has an UPnP plugin in development.
This should make it possible to use Musicbee to play it's library to Squeezebox players.
MusicBee's sole developer, (Steven) is currently active to iron out any problems with this new plugin.
So anybody interested in this could have a look, try it out, and help with giving Steven good input to perfect this plugin.
The MusicBee forum topic about this UPnP plugin is here:
(edit, you'll have to find it yourself, it seems I am not allowed to post a link here.)
The current plugin version is to be found in reply #46. (but that will probably change later on)
I hope there will be some enthusiastic input from other Squeezebox owners to help to perfect this.