Both the DFS and the ADFS did indeed reserve memory for themselves at startup, increasing PAGE; this memory was used to hold the current catalogue and other bookkeeping.
For adapting existing tape software two observations are helpful:
almost nothing on the BBC was a multiload; and
you can turf the [A]DFS out of memory if you don't intend to use it again.
It is rather straight-forward, technically speaking, to share both 3.5" floppy disks and their image files between a Classic Mac (e.g. MacOS System 7) and an Apple IIgs. This because both systems support ProDOS floppies and HFS floppies (in GS/OS), and both systems support image files for 800KB disks using DiskCopy 4.2. So, that solves most needs back ...
In reference to the comment "Any idea how the IIGS implements those Disk II motor detectors?"
There's this Technical Note #68: Tips for I/O Expansion Slot Card Design.
On the Apple IIGS, the Mega II select signal (/M2SEL) is used as the
enable to the slower, 1 MHz side of the system. It goes active (low)
whenever the 1 MHz side of RAM or ...
From the Apple IIgs Hardware Reference, Second Edition, pages 22-24:
Figure 2-6 - Speed register at SC036
Table 2-2 - Bits set in the Speed register
Description Disk II motor-on address detectors: To retain Apple II
peripheral compatibility, the motor-on detectors change the system
speed to 1.024 MHz whenever a Disk II motor-on ...