External diff viewer and stage/merge tool and git-gui?

Hi!

As I continue to introduce Git at my workplace, I got the question on how to integrate external diff tools with Git. I figured out a way to get it to work with the command-line "git-diff", by setting GIT_EXTERNAL_DIFF and hacking together a small batch file (yeah, this is on Windows) that opens the diff in the viewer. For myself, I would like to use tkdiff, whereas the rest of the team is used to another side-by-side diff viewer.

However, I cannot seem to find a way to do it from git-gui, which is the tool I have taught the rest of the team to use as their primary interface. Is there a way to view the currently selected difference in an external diff viewer?

Even better would be a way to allow that external tool to edit what is to be staged - we are having some problems with version-controlling sources in some of Borland's tools which have the bad habit of storing local configuration in the project files, and git-gui's "hunk" picker cannot be used to pick the relevant changes (we have to set "show less differences" to the lowest value to get a proper hunk view, and then git-gui cannot stage anything).