At present when an SBA goes into Survivability mode, users lose all RGS functionality. A good example is that a customer may use an RGS for the branch sites Reception.
Obviously SBA's are not powerful enough to run IVR's but there could be a "Backup Hunt Group" within the Response Group workflow.
E.g. Normal operation all RGS calls are dealt with via FE Pool (HG or IVR)
During WAN/Pool Down scenario the SBA can still deal with routing a basic Hunt Group and queuing users.
I know you could deploy a SE server but that isn't an option for some scenarios (size of office, cost). You could do some clever re-routing across the PSTN but it would be nice to have some sort of backup. I have also worked around this creating a dummy user registered to the SBA that "Sim-Rings" the RGS. You could sign in as that user during WAN/Pool down to pickup the inbound calls.
However it shouldn't be that complicated.