Of course, there is a fourth of "two other" possibilities:

(The third was just the previously posted patch, so two "other than the 
posted one") :-)

Make a script calling git-commit with "-F - -e" and pipe your generated 
template into it.

You considered that this script should parse -a, -i, -o, whatever, 
right? ;-)

The point is that a hook can use the index as prepared by git-commit.

Paolo