Are you talking physical dimensions, or just logic level compatibility? I was attempting to build your 1MB ISA board into a PLUS format perfboard to figure out the latter myself, but got sidetracked with other stuff. It would only need half the components (only needs to be a 384K board), because the EX/HX come with 256K onboard and are meant to expand up to 640K. They might be able to go past 640k to 1MB, which is the maximum the 8088 can address, but I don't know if the BIOS can recognize it.
The main issue with any expansion for the EX/HX is just making everything fit, combined with the Tandy RAM/DMA card also having the expansion headers for additional expansion cards. The space restrictions are even more tight for ISA cards with adapters.