Hi Mike,
SNP heuristics can't solve your problem without manual intervention. By design, it just proposes neither optimal nor feasible solution.
At our site, we used an hybrid approach using SNP and deployment heuristics with outbound quotas at plant (100% to DC and 0% to overflow warehouse) and inbound quotas at DC (100% from overflow warehouse and 0% plant). The replenishment of overflow warehouse is managed manually (stock transfers from DC and plant to overflow warehouse).
Thanks,
Rajesh