I need to access the virtual disks of a vm while it is in the BuildingMachine state. I need to assign a storage reservation policy on each disk so when the machine is provisioned the vm can be stored on the appropriate LUN.
Once you install the vRO customizations to your vRA environment and create a BuildingMachine workflow you can do pretty much whatever you want to the properties of the VM and they will be applied when the machine is being provisioned. There are several actions under the com.vmware.library.vcac module that will give you the basics on how to update properties on the VMs, including the storage reservation policies.
You can also hard code the reservation policies on a per disk basis in the blueprint for the machine, you can take a look at the reference guide on what properties need to be set for that.