Hi Moll,
> I don't know where I got it. Asimov or Trenco most likely. If you want it, 
> take all the spam out of my address and I'll mail you the disk image. 
> "Program Writer" and "GPLE" are such hybrid disks. DOS.SYSTEM is on 
> "Program Writer" as well as the ProDOS side of Extra-K.  The DOS version 
> of Extra-K is easier to find than the ProDOS version.

No probs,
Found it on 'Ground'.

Here is a link to a zip file with the corrected versions:
http://au.geocities.com/redskulldc/dos.system.zip
includes a pre-built hybrid disk with a pronto.system.

This is what I did:
Fire up applewin (I have the J version, but the latest is fine)
Put a blank pronto-dos image in drive #1
Go into the debugger
Set a breakpoint at $B747. this is normally the last instruction
before DOS3.3 is started. (True for DOS and Pronto-DOS, may
vary for some of the others, but it will be nearby).
Normally contains a JMP $9D84 instruction.
Hit G in the debugger to continue executing.
Boot the disk with PR#6
You may get a couple of breakpoint hits at $C6C6,$C6DC,$C6E6
ignore these by hitting G <ret> in the debugger.
When the 6502 gets to B747, change the PC register with:
R PC FF59<ret>
G <ret>
this will dump you in the monitor, with your chosen DOS fully loaded.
Move it out of the way with *5D00<9D00.BFFFM<ret>
continue executing DOS with:  *9D84G<ret>
save your dos image with ]BSAVE image,A$5D00,L$2300

From there I ran the new image through your program to create
the pronto-system file, copied the files to the hybrid disk, and
bob's your uncle.

Cheers,
Red