Well, everybody gets extra points for creative thinking here, but I believe that what I want to do is not technically possible with Sonar X2a, or possibly any other DAW, for that matter.
 
The problem with muting or automating volume on the metronome track as an only solution is that it will be the DAW defining where the downbeat is on the resumption of steady tempo, not me.  I may be able to work with that by getting the metronome volume sufficiently attenuated in the meandering bit but not so much I can't hear it, and then "working with" Sonar's click to restart on a mutually agreeable moment.  Still not ideal but it may work.
 
The problem with doing things in two separate takes is that it's doing it in two separate takes.  The feel of the musical meandering naturally resolving back into a steady tempo may be very difficult.  I'm not going to dismiss the idea of a cut and paste solution just yet though.
 
Thanks for all the suggestions everybody!

...or perhaps what you really need to do, is go (almost) completely old school.  Turn off the SONAR metronome.  Set your project tempo to match the tempo of an actual hardware metronome.  Record your "bed" with the real metronome in your headphones and stop it for the meandering part...then restart the metronome when you go atempo.  Then...if you need the project to match up with barlines for MIDI stuff or subsequent takes...do the (somewhat painful) tempo remapping procedure to match the project tempo with your actual performance.

Record separately / cut and paste makes it easy. I don't see an issue with the feel if you do it that way. If anything, that would work better as you wouldn't be trying to squeeze your meandering into a set period of time. Your downbeat back into tempo would happen wherever you wanted it to. 

Either that or set a guide as stated then overdub until its right. Most of the time people meander pretty much at the same speed every time when overdubbing.... It kind of gets its own tempo/feel without the  mathematics.Wouldn't take much to get it right, in which case the high hat suggestion by Bristol J would work just as it did for the Beatles in day in the life for instance.

Sorry..... I'm meandering ;-) 

Good luck

Bibs

@Dan and Bibs,
 
OK, now I'm just plain impressed.  An old school hardware metronome - brilliant.  Never occured to me.  And an iteratively tunable cut and paste for a guide track.  I'm gonna try to integrate these two ideas and hopefully I won't confuse or injure myself in the process.  Thank you again.

The problem with not using Sonars clock is you won't be able to do any MIDI editing with quantizing. 

Cactus Music


The problem with not using Sonars clock is you won't be able to do any MIDI editing with quantizing. 

I should be able to if I go back and build a tempo map to my guide tracks, right?

Bristol_Jonesey


Another solution would be to not use the metronome - use a hi hat in Session Drummer - you can place the hits wherever you want

Maybe use a footswitch to actuate the hi hat?  Is that the idea, Jonesey?  Then go back later and tempo map that hi hat?

No, I was suggesting you go into the prv and manually add a hit wherever you want the 'metronome' to sound, up to the point where your "tempo-less musical meandering" section starts.

Then you can do your meandering, for however many bars that might be.

Then when the tempo picks up to a regular beat again you can insert more Hi-Hat hits to sync into.

  dear wizard, sorry for off-topic, but i checked out your video "yeah it's good", and it is indeed really really good. totally liked the effects. could you let me know the software you used for the video? i create my videos with the windows move maker, but these effects are not included here... thanks - tom