Some random thoughts about the proposal.
Why not to use a single function to discover the gateway and all the possible methods that need L2 solutions?
You could user ERC1820 or ERC165 to publish the interface with a single function gateway() view returns(string).
Another option is use ReverseENS to discover the gateway.
The gateway may have a standard call to query all the methods that require L2 data.
I would fill confortable to standarize a raw bytes l2data parameter at the end of the method for all the L2 data when required.
The query could just be the call with that parameter empty, and the result the l2data. The client will just rebuild the calldata with the returned l2data.
I don’t like clogging the smart contracts by doubling the number of methods.