Hi, I'm the person behind http://code.google.com/p/prs500fs/
It looks like this is exactly the thing I'm doing (and I was targeting Mac as a first release platform too). Although due to the lack of time I'm still in the "tools" stage - just finished libusb C++ wrappers by today - wanted to start coding prs protocol tomorrow evening.
Any chance to see the source code of scotty1024 filesystem? =)
If so, then perhaps http://code.google.com/p/prs500fs/
is not needed at all..
If not, then we could still benefit by sharing bugs information etc.