I've read a few articles about the ScuzzyGraph. The first thing to note is that a device driver is essential because the Mac does not normally send video signals along the SCSI bus. It is likely to be a system software driver (so that it can be updated) and to be dependent on specific system versions (which possibly excludes later versions of System 6).

It works by hooking into the QuickDraw calls that generate a shaded pattern, substituting them for a colour block. Thus, it can create colour versions of MacPaint, MacDraw, Excel chart documents etc.