e.g. Users in orgA can add as much as 16gb of memory to their VMs, however orgB can only set a max of 8gb. I'm wondering if this is possible WITHOUT using vCAC or vRAC.
It is possible to set in the allocation model settings - VMware vCloud Director Documentation Center
Yes you can by using a mix of AMQP messages and Orchestrator. See here: CPU and Memory Limit enforcement for vCloud Director