I did some more digging on the multikey feature that appear to be available since there are some definitions for it in the kernel keymap file.
So far, from what I can tell is that OPIE seems to support it since there are some extra keymaps for multikey in OPIE and also a readme file for it, whereas I couldn't find any further info for it in GPE, so I take it that GPE does not support multikey? Am I correct or did I miss something?