I sent to foobar the file I was to preview.
Kind-of on-topic (since this thread is still in wishlist) :
+1 for some preview/dual-output functionality within MusicBee.
Kind-of off-topic (only slightly) :
Have you considered reversing your workflow?
So foobar2000 would be the player that outputs to the main speakers/audience, and MusicBee gets purposed to previewing, browsing your library, and sending the tracks you want to play to foobar2000?
Two programs running at the same time here has some benefit. You'll only have to look at foobar2000's panel to see what your audience will be listening to (next).
And you can mess around with MB and it's library without worrying the audience will notice or that playback will be interupted.