OK here is the plugin.
 
It works as expected, with one caveat... If you solo the track bus, Sonar will not solo the tracks feeding into the track bus. To get around this, solo the whole folder, or group the solo buttons together.
 
To use:

• Unzip the files.
• Copy either/both the 32-bit or 64-bit files somewhere into your VST3 folders.
• Let Sonar do a scan.
• The plugin shows up under Plugins > Audio FX > VST3 > Sidechain Mixer (or wherever you put it).
• Drag/drop the plugin into the FX bin of the track that you want to convert to a bus.
• Set the output of any other track to "Sidechain Mixer(Input 2)-[track bus name].

 
I only tested the 64-bit version so let me know if the 32-bit version doesn't work.
 
BTW you can also send sends into the track bus.

Holy crap that's awesome. I can't wait to try this out SilkTone. I may be more excited about this than I am for the new version of Sonar...
 
Thank you,
 
Dean

Noel Borthwick [Cakewalk]
I understand the request but why is it important? :)

Parallel processing.

One weird thing I noticed...
 
I wasn't sure what the plugin will do with mono tracks, so I played around with the track interleave buttons. If the 1st track going into the plugin is mono, all subsequent tracks will be treated as mono (???). However if any of the non-first track is mono, then you get the expected behavior.
 
Maybe Noel knows why it is acting this way. I don't think it is a big deal as it is easy to make sure at least the very 1st track is stereo.

Wow, that was fast. Lots of thanks, SilkTone! This will be a nice improvement to my workflow.
Nice graphics, by the way :)
 

SilkTone
One weird thing I noticed...
I wasn't sure what the plugin will do with mono tracks, so I played around with the track interleave buttons. If the 1st track going into the plugin is mono, all subsequent tracks will be treated as mono (???).

Just out of curiousity... Tested it with a Sonitus gate sidechain input. The behaviour is totally the same. So it's just one of those things we'll need to remember about sidechaining. No biggie.

Mod Bod

Noel Borthwick [Cakewalk]
I understand the request but why is it important? :)

Parallel processing.

I may be missing out something, but won't be the current bus-routing manner with pre/post-fader sends more natural for parallel processing? After all, it would be nice to have send amounts on one source track (and automate them on that very track) than to have to hunt input gains on the receiving tracks.

Note that you need to set the PC to Post FX bin if you want to use it in the signal path.
 
Alternatively there are also other routing options... You can add an FX bin into the PC (move it to the top), and use that as the input from other tracks/sends. You can also add multiple FX bins into the PC, and feed different tracks/sends into different points along the PC. Not sure if it can be useful, but it seems to work.

Well there you have it, ladies and gentlemen.  A solution to the need of the few.  
 
I took three guitar tracks, set them for stereo interleave and routed them via post fader sends to another track that had Sidechain Mixer in the first slot of the FX bin.  I switched the output on the source tracks to NONE and set the destination track's output to my ALL GUITARS bus and I could control the overall volume of the Electric guitars with the new destination track Electric Guitar Mix.  I could blend the individual guitars via the track fader or the send levels.  Of course my preference is the track fader.
 
I also verified that if I set the output of the source track and the output of the destination track to the same bus, I get a parallel copy.  I confirmed to myself that another FX added to the bin after Sidchain Mixer would work properly and it did.  I placed Channel Tools in the Electric Guitar Mix and flipped the polarity on the channels and as I adjusted the fader on the EG Mix, the source track and the destination track started to cancel.  I never got a full cancellation because I didn't match the levels as they should be but I didn't hear any change in tone as the two tracks started to cancel and that gave me confidence that there is no phase issues.  At least I didn't hear any.
 
I did get some phase issues when I engaged a second effect in the parallel tracks FX bin but it mysteriously went away after I started looking for the cause.  I'm thinking it was a temporary ADC issue and when I restarted the audio engine, it synced back up properly.
 
Good job, Silk Tone. 
 
EDIT:  Just for giggles, I tried arming record mode and recording to the destination track.  As expected, no recording.  No, Freeze doesn't work either.  

SilkTone
OK here is the plugin.

Thanks!

Mod Bod

Noel Borthwick [Cakewalk]
I understand the request but why is it important? :)

Parallel processing.

I was referring to this statement "It would be important that the routing be accomplished via the input of the destination track (or some means other than the source track output)." 
 
Why is it important to you that routing is accomplished via the input rather than the output as it is today for sends to buses. I woudn't want to change the UI paradigm just for this.