I agree, would be better to have a distinct SPS proxy setting for that and a set_sps_proxy operation for the RPC nodes. This would require to add an additional field like sps_proxy in the get_accounts result and the list_accounts method would need to be updated to be able to search for accounts ordered by sps_proxy.

I hope something like this is planned for a future hardfork.
At least we (all) now know how it works.