I guess when you say ‘if the 5V was still available’ you mean to provide 5V from an external source like microUSB cable, and not use the internal 5V provided by OSPi? There isn’t a very easy way to achieve this, probably the simplest is to remove the PTC fuse (the green component on close to the bottom of the circuit board. Once the fuse is removed, the internal 5V is cut off from the rest of the circuit.
On a related note: OSPi v1.4 has a built-in relay and all three relay pins (CO, NO, NC) are mapped out on the board. If you just need one relay, it’s pretty convenient to use the built-in relay (controlled by GPIO pin 15).