Welcome.

In order to upgrade software, you first need a serial port, which might not be available on your Mac. I believe that David has said that at least some of the USB-Serial adapters work on Macs.

Once you're over that hurdle, it's easiest to do it with a Mac using Virtual PC (especially if you already have it), but I'm sure that it could be accomplished via some other methods as well. Splitting the .upgrade file and using the upload program to upload the pieces individually might work, but hasn't been tried before AFAIK, and would be complicated, although all the pieces should be there to do it.