That's really the key here.... You need a patched copy of HD SC Setup on your Plus. With the patch, it no longer checks for the Apple ROM and will accept to format any accessible drive. In fact it will even format a Zip disk as though it were a hard drive!
Check out: http://www.euronet.nl/users/ernstoud/patch.html
Now, the challenge will be getting it onto the Plus in the first place.... A Mac with a superdrive would be a huge help. Given what you've got to work with I'm not sure what to suggest. :-/