As you note, the router id is explictly configured in your scenario, so it will not change. The impact of the interface being down will be that the router will not be reachable as an ospf neighbor.
But in your design, if the tunnel is the only way the neighbor relationship can establish, then this might not matter.
That is why Ron is saying it is safer to use an ip address that is always up as the router id. Since the loopback can never go down, you can always respond to those neighbor requests regardless of which interface receives the traffic. Even if the primary path interface is down.