The Hitchhiker's Guide To GEOS contains no section on zeropage usage. However the section on public GEOS variables shows variables at locations in the range $53 - $78.
Some reverse engineering on existing applications shows that the GEOS pseudoregisters r0 - r15 are mapped to the locations $00 - $1F. This is very plausible as they are mapped to the locations $02 - $21 on machines with a 6510 (where the location $00 and $01 can't be used).
An Apple II emulator instrumented to create a zeropage usage statistics seems to indicate that the locations $80 - $FF are not used by the GEOS Kernal. Therefore the location $80 was choosen as base address of the cc65 zeropage memory area.