Funny, I was about to be lazy and say I could create a keyboard macro that brings up the Xcode Refactoring window, does a Copy, and closes the window, and that would suffice for many of my needs. But having a bunch of work already done by you makes the problem much more interesting.

But that's a command-line tool, not a service. No problem: ThisService makes it easy to turn that command-line tool into a service, and then to make it distributable. Here's the service: http://boredzo.org/stuph/ExtractSelectors.zip

The tool I wrote is not exactly what you wanted (unsurprisingly, considering it pre-dates this post by nearly four and three-quarter years). For one thing, it only works on declarations, not message expressions nor fragments thereof. But triple-click-and-drag makes selecting the whole declaration easy enough. The other problem is that it acts as a filter, replacing the selection with its output instead of copying it to the general pasteboard. That problem would not be hard to solve, and I'd welcome a patch.

Nonetheless, I hope you find it useful.