I know this is possible, and probably not even difficult, but I'm struggling.  How do I record to a track that is just the output of say, a reverb bus?

Hi. On the latest versions, you could route / send the bus to an AUX track or Patch Point? Hope it helps.
All the best.

Here's a video using aux tracks: https://www.youtube.com/watch?v=rCAlFThGvH8 It doesn't use a buss as an example but it's a similar set up.
 
Rocky

Yes, Aux tracks can record the output of a bus however, depending on what you are doing, it may be simpler to just do choose "Busses" in the File > Export Audio dialog, select the bus you want, Export then Import the file back into the project (I usually do that via the Media Browser > Right Click the file > Insert at Now Time).
 
That circumvents all the routing/track setup involved in actually recording the bus with Aux tracks and does the same thing. The only situation where I could see it being a problem is if you are trying to record the bus output of live inputs (as in you want to record the bus output as you track something else into the project).
 
Cheers.

Nice vid, and good option from Beepster.  I'm set.  Thx all. Let the reverb drenching commence.