OK this info is useful, so I need two Macs, blimey!
Well... not necessarily. You will at least need an Intel Mac. As for PPC you could find a trusting fellow to compile and test anything in the meantime, who could then give you the PPC binary for you to make a universal binary out of.
Mark himself mentioned that there may be a quick hack to fool BlitzMax into compiling a PPC binary on an Intel Mac, although I'm not sure what he may be doing with this thought, as no more has been mentioned. Of course you would still need to test the program on a PPC Mac somehow.
Fortunately, I don't have this predicament, as I have both a PPC and Intel based Mac. As such, I would be willing to help in the future should you require it.