THambrecht
Panning MIDI is only possible if the synth (Hardware or VST) is a STEREO synth and receives messages for panorama.
Otherwise the pan-controll has no function. For example the novation bass station (hardware) has only a MONO out.
Its up to your synthesizer.

In which case you can pan in Sonar. But since he's talking count-ins and basic midi files it seems he's using a multi timbral GM style unit - I have never seen one of those in mono.

Thanks everyone for their suggestions and feedback.  It has been very helpful.
 
Just coming off vacation, so when I get back to working with SONAR tomorrow (Tuesday), I'll try CC10 - which I don't know why I hadn't thought of myself, duh.  I'll need to put count-ins and such on a separate track of course, so there's a lot of editing in my future.  Pretty sure this thread will get me where I want to go.  Thanks again!

In addition to a separate track, you'll likely need either a separate sound module or an additional patch (loaded on a different MIDI channel) into whatever multitimbral module you're using.

The easiest way to do this, if you want the audio to remain in stereo, is to use an interface that has more than two outputs.

Route the stereo audio to a pair of outputs and the click to another, which is then routed by the mixer to just the monitors.

Though the advisability of running a PA in stereo is debatable - often large sections of the audience will be hearing much more of one "side" of the stereo than the other, unless the nearest audience members to the PA speakers are somewhat further away from the speakers than the distance between the speakers. If the PA is run in mono then send the backing audio to one interface output and the click to the other, then again at the mixer send the audio to PA and foldback and click to foldback only.

PS - MIDI is neither stereo or mono, it's just instructions telling something that understands the instructions what to do. As well as telling a synth to pan its output you can also pan the resulting audio using Sonar's ordinary panning controls, or collapse a synth's stereo output to mono.

Sometimes I would make the count-in a short musical phrase instead of a click so it was OK for people to hear it (e.g., a bass walk in, short guitar line, light drum fill).  Instead of a straight click track, I would have a hi-hat, shaker, or some other instrument keeping the time.
 
I understand you don't want to rearrange all your music: just some ideas for your consideration.