I just found a package on AmiNet called diskimage.lha which sounds like it lets you mount .ADFs and compressed disk images as virtual disks with device names like DI0:
I haven't yet tried it, but supposedly it works on Amiga OS versions 2.0.4 and newer. I'm going to try it on my A2000 or A1200 soon. They're both taken apart for maintenance right now.
Theres some other package on AmiNet that claims to do the same thing on OS4.
