What about a Virtual Machine? I've found VMWare Fusion does a good job with Mac guests. Snow Leopard works quite well. You could even do it from a non-Mac host if you don't mind applying well-known patches to VMWare.
I think the best solution is to just keep both, I have all the source code for all my projects since maybe 2008. But I also keep a lot of screenshot, if I want to refer to an old project I go to those screenshots (or even better, videos) in 99.9% of the cases.