I was searching for quite a while, but I could not find any sources that could help me write my own ProChannel plugin. Does anybody know if there is some SDK available, or perhaps simple example "gain plugin" from which one could expand?

You can do quite a lot with PC FX chains. Re-skin them, change the knobs, etc. You can insert any plug in.

I have a few for Guitar Rig and a Fairchild compressor, amongst others.

That's one good suggestion, thanks, but what I intend to do has to be its own plugin, and unfortunately FX chains will not do.

In that case I think you would have to contact Cakewalk directly.

They have obviously made it available to some third party developers like Boz and Softybe, but am not sure about a private customer.

You never know, if they agree and like your work, we may all benefit with a new module.

I know a tuner would be greatly appreciated by many..

AFAIK, PC modules are just "containers" for vsts, so the developer has already built the plug in, and just "re-skins" with Cake's PC module.

Good luck.