I think the modular approach is the way to go -- I would be grateful for contributions... after seeing what you've done, I think you should really have the 'official' github repo of Shellista. If that doesn't work out, I've added you as a collaborator to my repo, so have at it!
My only desire would be to add that script that grabs the zip directly into the ShellExt.py file... I know it probably uglifies things a bit, but it would be important for a nice user experience for people new to Shellista/Pythonista. I just got Pythonista a few days ago, and I think I might have been scratching my head a bit wondering how to get all those files in there without Git built in... the original Shellista was a simple copy-paste job (you are right-- it's a simple script, but I'm a little absent minded-- that probably wouldn't have been my first thought! :) )
I found a pure Python implementation of Git called <a href='http://www.samba.org/~jelmer/dulwich/docs/'>Dulwich</a>. There is also a wrapper to that called <a href='https://pypi.python.org/pypi/gittle/0.3.0'>Gittle</a>... I used both of them. Everything except the git push
was very simple. With that, I was having all sorts of issues with the authentication. I believe this is because of versioning of Gittle/Dulwich... they seem to be out of sync. I was not able to pair the correct versions, so I ended up writing a monkeypatch for Gittle's push_to() method, which worked in the end. Not ideal at all, but it practically speaking, it works for now.