Change multipathing for most but not all LUNs

I have ESXi5.0U3 hosts and EMC Clariion storage, connected via dual fabric/dual HBAs.

I want to change the multipathing from MRU/Fixed to Round Robin to try to balance the load across the two HBAs.

I believe that if I change the default PSP to Round Robin this will change the multipathing for all LUNs (after a host reboot).

However, I have some Windows Cluster VMs using RDMs for shared disks, and I believe that these must not be configured with Round Robin PSP.

So what I ideally want to do is to configure each of the RDM LUNs to use the MRU PSP, and then change the default PSP to do all the other ones. Is that possible?

Thanks in advance for any help/advice/guidance/links/etc.!