There is no official or supported plugin system for the Finder. Starting with OS X 10.6, you will need to inject code into the Finder process and override objective C methods in the Finder process.
I've done this for a proprietary project. I can tell you that the reason that there are no examples or tutorials for this is because it is a significantly difficult and time consuming development task. For this reason, there's plenty of incentive for individuals or organizations who have accomplished this to guard the specifics of their process closely.
If there's any way at all that you can accomplish your goal using the Services API, do it. Writing a Finder plugin will take you 1-2 solid months of painstaking development and reasonably deep knowledge of C and Objective-C internals.
If you're still convinced that you want do to this, grab mach_star. Good luck.