The proposal document is linked in the email. ZmnSCPxj mentions that the locking phase described in the proposal is not safe to perform from recipient to sender. R and S are essentially the same entity, and multi-stage contracts can be done from R to S, but initial establishment has to be from S to R. There may be other issues with the overall setup, and ZmnSCPxj is considering what would happen if contracts were established correctly from S to R. The onion packet can transfer secret data from S to R without needing separate communication.