I've recently discovered that there is a homepage
for the EPUB discussion group of IDPF, and it's more up-to-date than the the IDPF forums. Just yesterday I wrote to ask if it was possible for the general public to participate (at that time, there was no way to create an account), and I was replied that that feature was being implemented that very evening (not because of my asking, though)
The result is, it's now possible to create an account and participate posting comments or issues there. Anyone wants to suggest the development of a specification for ePUB dictionaries?