Hi Dave,
The mentalray jobtype does indeed support standalone satellite renders, using arbitrarily many nodes. It will automatically allocate nodes and use the "-hosts" option to the "mentalrayrender" command.
For example, if you want to use 25 nodes, then on submission, you will want to set the "CPUs" field to, 25, and then also set the "reservations" field to "host.processors=1+" to make sure that each node will only take one subjob.