I wrote this small patch because I needed this very feature.
I just began reading the source so I may have done it wrong, please let me know if it is the case, I'll be glad to follow your advices/conventions.
Ability to restrict a plugin to certain chans only
I'm not sure I want it to be part of Cinch's core, because it's a very specific use case. Other people might want to limit it to users, or users in a channel, or forbid it for some channels only, or forbid it for some users in some channel etc.
I'd suggest using an ACL extension, like the following: https://gist.github.com/3857485