I am envisioning a tool to help with MIDI key velocity editing. I do a lot of work with "metrical accents" and wish to make that a separate layer that is measure-based and is summed with a basic track volume envelope. Is there a technical guide to aide with programming a plugin for Sonar?

I would think writing a plug for SONAR is no different than writing one for Reaper, Cubase or Studio One. And for tha matter, no different than for Logic (or any other Mac only DAW).
 
I believe Steinberg probably publishes a VST/VSTi standards document.

Thanks, I'm checking out the Steinberg site now. Was hoping that since what I want to do only involves MIDI editing, not playback, it would not be too involved. We'll see.

Cakewalk used to have own format DXi2 SDK. It is hard to download now since Cakewalk has abandoned DevXChange site. So I guess VST is the only way to go.

The DevExhange DX page and downloads are still available through the Wayback machine.
 
https://web.archive.org/w...e/article.aspx?aid=112