Thankfully, the awesomeness of Git and Github came to the rescue - I simply forked the project, looked back through the commit log, found the commit that introduced Rails, then "re-mastered" my fork so that commit was the new HEAD. And how did i do all that?
1. Fork the project on Github.
2. git clone firstname.lastname@example.org:$USERNAME/$REPO.git
3. git log
- find relevant commit, note down first 8 chars of the commit SHA. Lets say that was '12345678'
4. git reset --hard 12345678
5. git push origin master -f
That's it - you've now effectively taken the forked repo back in time.