FYI, this seems to be a very odd entry in the Apple Knowledge Base:

http://docs.info.apple.com/article.html?artnum=8757

and seems to suggest something that is not supposed to be possible. It makes me wonder if an HD20 could be formatted on a Mac Plus with System 2.0/4.1, basically resulting in a 20MB MFS volume that could then be used with a 64K ROM Mac.

Otherwise, why have a tech note about it? I will test and report back.