I wouldn't advise trying it on irreplacable MD recordings - you may want to test it by recording an MD from a CD source and then titling. It's unlikely to cause any problems on your powerbook, though.
As for the incompatibility, my belief is that Sony quite simply doesn't need to support...
http://www.sorted.org/~pete/Xmd/ - Abandoned 12/2002
http://libnetmd.sourceforge.net/ - Abandoned 3/2004
http://www.pdr.cx/projects/gnetmd/main/ - Abandoned 3/2003
These are the ones I was tracking before I gave up, as you can see even the most recent one has been dead for over a year, and...
SonicStage has always been PC-only and no plans have been announced for an OSX port. Due to the proprietary nature of the ATRAC3 format, it is astoundingly unlikely that any third party support would be forthcoming.
Here's the list:
Pretty short, hm?
NetMD isn't compatible with anything but Windows 2000/XP. There was a short-lived project to get it working on linux, but as far as I know nothing came of it.