Need to patch DOS 3.3 for H/D

[dmanzer@wimsey.com (War Lib'y Canada)]

I'm working on a custom (ie, low-cost) PC-based file server for
Apple II. (Details for the project were posted here this
spring, with reference to ProDos).

To make this work with DOS 3.3 I want to patch RWTS (read/write
track/sector), using some of the unused space inside DOS 3.3
commonly used for patches. There would be one config byte
allocated such that Bit Number = Slot Number; 0 = Disk ][ card,  
1 = custom H/D card.

Then when RWTS is called it jumps to the patch, which determines
if a floppy disk or the hard drive is being addressed. For a
floppy, control returns to RWTS as normal; H/D access uses a
special entry point in the custom card's firmware, passing the
addressed sector as a parameter.

Does this seem like a reasonable approach? Any comments (or, even
better, any hard info?) I'm using Don Worth's _Beneath Apple DOS_
which is helpful. I also need to deal with write protect, either
by disabling this in DOS or by controlling it from the IBM (I'm
using a parallel printer port on that end).

BTW I don't plan to boot the system in the normal way; instead it
would auto-load via commands from the IBM. A file server
application at that end would include keyboard commands to select
operating system and mount/demount volumes.

Regards, D.M.

ps. _Some_ of this stuff is already working. However, the project
was (temporarily, I hope) set back when I kept getting the message
"UNABLE TO LOAD PRODOS" on boot up.