Here's what I have been able to cull together so far:
(1) The Z80 BOOT PROM code (version 1.2 at least, which is the source listing I have) appears to probe the Winchester, for boot e.g.
- try to read Winchester
- check result
- if error, try floppy
- if ok, check Winchester label
- if bad label, try floppy
- if good label load system and boot??? (this part is unclear to me)
(2) Not all of the v1050 CP/M BIOS versions appear to support the Winchester. Versions up to 1.1 do not appear to contain the WINCH.ASM module which I am assuming is required for support. I do not have access to version 1.2, but I see this module showing up starting in version 1.3.
I would assume that with the 1.2 BOOT PROM and the 1.4 CP/M BIOS I could support the Winchester (barring any bugginess - I believe that at a minimum there is at least a v2.0 of the CP/M BIOS [which I've never been able to obtain images for] and not sure on the BOOT PROM).
It seems that there is likely some additional software to format, partition, install, etc. that I do not currently possess.