One solution suggested by Joe K:
- User sends hash for timestamp
- ACG Ethereum A/c send ether = 21000 gwei to user's a/c
- User's a/c automatically uses this ether = 21000 gwei as payment to publish transaction
The only drawback is that we are essentially conducting two transactions i.e. ACG to User and User to Blockchain. Therefore, we will end up paying 42000 gwei