I think we should be able to edit Classifications and Custom Properties attached to Building Materials within a schedule. Personally i feet this ability should be at high important as this is about as BIM as you can get within archicad.
In the Element Schedule when adding a field for a classification the option at the far right of the field has the selection between "show Classification of" grey out not letting you drill down to the building materials classifications. ... and i assume there i no way to drill down to a building materials custom properties in the Element Schedule.
In the Components Schedule you can easily set up a schedule to display a Building Materials Classifications and Custom Properties, but you can not edit these from the schedule.
I assume this is a work in progress - Please put this up near the top of the list.
Self Employed - Modeling, Estimating, Construction
AMD Ryzen 9 5900X 12-Core Processor
3701 Mhz, 12 Core(s), 24 Logical Processor(s)
(RAM) 128 GB
NVIDIA RTX A2000