I have msysgit installed, with OpenSSH. I am connecting to a gitosis repo. From the git bash, I have created a .profile file that runs ssh-agent (if not already
You could wrap your git executable with a script that sources your .profile, causing the ssh-agent environment variables to be loaded.
Either put a script called git in a directory earlier in your path than the real git, or configure the git extensions to call your wrapper in place of the real git.