Got it!
And I think I traced the issue.
The driver found via
http://tech.prolific.com.tw/visitor/v_fileBrw.asp gives the right one, just needed to be installed, and viola, it goes.
Why it worked on my G3? I remembered I borrowed my friend's Garmin eTrex and his Serial->USB adaptor. My hunch is that it was the same chipset (his of course included mac drivers on CD). Thanks for the help!