The conformance tests are open source and will be available from day 1 ;)
As far as participating in the design of the standard, are you really saying you'd want everyone and their mother to have their word? I think it's best to leave this to GPU vendors and engine implementers, as they're the ones who know how it can work.
APIs such as DirectX or Metal receive just as much or more input from developers, albeit mostly through back channels or explicit reach out by Microsoft, Apple, etc.
[1] https://www.khronos.org/members/join/