I picked up an ATonce Plus yesterday ( along with some trapdoor RAM )
Thankfully the disk images are still available at http://amiga.resource.cx/expde/atonceplus .
Today I've popped it into my trusty A500+
Since I don't have a hard drive, I just started the emulation environment as it is.
ATonce BIOS reports 640k base memory, access to Mouse, Serial, Parallel port and VGA video.
Volkov commander ( sorry, my studio is too bright )
It is awfully slow, a lot slower than my PPC512. I don't think it would make much difference if the utilities were installed on a hard drive.
Nonetheless it's a nice working piece of obscurity.
According to the manual, it should support external disk drives, but it just doesn't