So long as all of the Assets are < 4GB, that external should be OK. PrPro will ask you "Where is file _____ ?" but will allow you to navigate to the new location. Now, I am not sure if one can set drive letters on a Mac, but that might simplify things a bit.
Archiving your Project with Project Manager, to that external, and then testing it, will likely be the best route, but do check the archived Project carefully.
One consideration will be whether any PC-only Effects, or Transitions were used in the Project. There are several of each, that do not exist on the Mac. You might want to check those out. I only have a link to PrPro CS4 differences between Mac and PC, and CS 5.5 will probably be different.
Appreciate the advice Bill! I'll give the archiving a try. The assets are all less that 4GB, as I already transfered those from the Mac to my PC. Now, if I can simply open a sequence on the Mac once a week to demonstrate what I'm doing that will be enough for this project. I'll also create some smaller test projects to see how compatible it is going back and forth. I'm hoping it doesn't take the Macs forever to render, as they aren't CUDA supported and several years old.
Just make sure that you do not use any of the "missing Effects and Transitions" on the PC, as they will not be available on the Mac. Maybe Todd, or someone else has an updated list of which Effects and Transitions are platform-centric? Like I said, the one that I have is now 1.5 versions out of date.
Good luck, and it should work fine, once you get past any drive letter thing (the links to Assets are absolute Paths, so everything must match up perfectly).