Hi @jamesmorgan,
Welcome to the community forum 
Just to confirm, do you want to have multiple trusted signers, to sign your users relayed call parameters (https://docs.openzeppelin.com/contracts/2.x/gsn-strategies)?
I can understand wanting to potentially extend GSNRecipientSignature to add an admin role that could set the current trusted signer.
I was curious to understand why you might need multiple trusted signers?
I recommend looking at the following tutorial (if you haven’t alread): Advanced GSN: GSNRecipientSignature.sol