I don't think STP1 has a way to prevent MSC1 from sending traffic to it.
b/m would work in my view:

1. MSC1 will always route traffic to STP1.
2. Primary route from STP1(for HLR GT) should be towards the DPC of STP3 or 4.
3. Secondary route from STP1(for HLR GT) should be towards DPC of STP2.

so if primary route towards STP3 or 4 is down, secondary route towards STP2 will become active.