Is it possible to add alternate SCM options in Xcode 3?
In the latest beta of XCode 4, there's a built-in support for Git, so it's possible to add other SCM.

Maybe Apple will also incorporate Mercurial support as well.

They are several requests - XCode should support Mercurial as well as Git., Xcode 4 should support Mercurial - to put Mercurial support into XCode.

So if you can't find a solution now, I'd suggest you to wait until XCode 4 goes out officially.



By the way, if you need or want a nice Mac OS X client for Mercurial, you should try MacHg.