Hi Michael,
MSA can be used in a standby like fashion. It is described in the Administration guide volume 1 under Manage Replicated Objects using Multisite Availability.
Here is a link the example on setting up a primary with 2 backups. It can easily be modified to your scenario:
The main difference between this and warmstandby is that you will not have the switch active command to finish replication in one direction before replicating in the other direction. You will have to manage the timing yourself. For example, you will have to make sure the queues to the replicate are caught up(quiesced) before your move your users to the standby copy and begin replicating in the other direction.
Dale Newth
SAP Active Global Support