Is there no way to point a reservation to multiple compute resources?
Unfortunately not, and this is one of inherent limitations of vRA which they improved upon in CAS with the concept of projects. A reservation is still a 1:1 mapping between a compute resource, because a reservation cannot track multiple resources across different clusters.
You will have to create multiple reservations for each compute resource and assign them to Reservation Policies. You can then programmatically assign the approprate reservation by changning the ReservationPolicyId during the requested subscription event.