We're discussing this with Valve. Naturally, we'd love to give Steam keys to our itch.io customers if Valve allows it, but right now, it seems that even promising a Steam key based on an itch.io purchase when the game isn't available on Steam appears to be a violation of the Steam Terms of Service -- and we don't want to make any promises we can't be 100% sure we can deliver on.
So, we've requested clarification from Valve, and we'll let you know what we find out.