The icd adram does make use of some of those areas. I've always thought of putting together a max ram a500: 2mb chip, 8mb fast, 1.25(or so) from adram : total of 11.25mb in an a500 
I had 2mb chip, 2mb fast & 1.5mb slow back in the day.
The 2mb chip ram upgrade I had didn't use any trapdoor or motherboard ram. Alot of them only have 1mb and you then you lose out on some slow ram.
