https://www.dropbox.com/s/3vrp9yk6d7d5f3j/Open%20Diff%20in%2...

I haven't gotten around to making it more generic (right now, it's really only supporting iTerm, but it shouldn't be too hard to do the equivalent for Terminal as well).

I'm using Kaleidoscope for my git diffs, but you can edit the first action to being whatever command you want.

Hopefully that works for you, but let me know if you have any questions :)