If anyone's still interested in this, I recently had need of mdvtool for a DIY microdrive emulator I'm working on (more about that in the future).
Anyway, I've fixed the Zip file import and added a 'create' command to create blank .mdv files, as well as squishing a few other bugs (but probably not all of them!)
I've attached a zip file of the source code if anyone wants it. You will need libzip to compile.
Jason.