We would just need someone with a large npub to kick off a working group thread by publishing a kind 30817 (you can do that from my client) event with a title like "Silent Payment Bitcoin Zaps" and a short text explaining what the goal is. The actual spec could be derived from the discussion or added in a fork and then merged back in. It would be the first Nostr Core spec written and developed on Nostr, @fiatjaf