Er, too much hassle to make a pretend DH0 !
How about just creating a folder (in windows) called `workbench` (dh0:)....
Then copy all the files you need to boot into that. you may have to first boot from a pretend floppy, to configure the OS...
This is what i did, (copying by hand, and not using the os3.9 installer), and it works fine for me...I have BTW a folder with all my ADF program files in another folder... :-D