Ok, I hadn't actually tried this before but yes it does sound like it should work.
You're right that the bus doesn't show up unless it has a track routed to it!
You say 'you tried adding one, but still nothing'. It worked for me, add an empty audio track routed to the metronome bus, and it becomes bouncable.
I'm writing this as I test stuff, and I see that it doesn't actually have the metronome there after bouncing. I guess that's what you meant.
If only Sonar had the feature of being able to select a bus as an audio track input and actually record it..
Bouncing the entire mix doesn't work either...So it's not just the metronome. I guess anything that isn't actually THERE (i.e. a live input) can't get bounced.
http://forum.cakewalk.com/tm.aspx?m=1181984 Seems this thread confirms that this can't actually be done...
Save using a drum VST and having to somehow 'make' the metronome, you could either take the hardware approach (route the metronome bus out of your interface and record it back in via a cable), or a simpler way:
Select 'Stereo mix' aka 'What U Hear' as the input of a track, and then solo the metronome and record. That records any noise that your system is outputting. If you're using an interface, you probably need to disable it and be using Sonar just on Windows to be able to get that input up.
Bit of a joke really...
Sorry for the false information - I just assumed that it would work because it really should!