Hey @zep! Unfortunately Relayer does not support HD-like addresses, and we are limited in building that feature since it's not supported by the underlying KMS solution we use. One option would be to spin up one relayer per user, but this would require a custom plan which may end up being quite expensive (I can contact you with our sales team though if you want to discus).
A simpler solution may be to just spin up smart wallets for your users. You can write a simple contract that only has logic for forwarding the erc20s it holds into a separate contract, and deploy one per each user you work with. You can even delay deploying it until there are actually funds to sweep, and use some CREATE2 magic to know the deployment address before deploying.
With this, you can have a single relayer that can call sweep on all smart accounts. Even better, since each user's wallet is not an EOA but a contract, you can easily batch multiple transfers on a single tx to save gas.