Market making on RELAY (and RELAY Classic) is possible using the RELAY Classic API, or the standard RELAY Classic 0x SRA. Please note that while RELAY does not show open book orders, orders on RELAY Classic are also available on RELAY.
Orders that are on RELAY and RELAY Classic are also picked up by other relayers and pushed to 0x mesh.
For information on the RELAY API, please see https://developers.radarrelay.com/
For information on the RELAY 0x SRA, please see https://developers.radarrelay.com/api/0x-sra
For information on the 0x mesh, please see https://github.com/0xProject/0x-mesh