I had another look at that crack in a different light - it's actually the wood underneath the paint has split along the grain when the pin got hammered in nearby. So it's a vertical split running from the bridge pin, to the closest edge of that part of the wood.

I've put some more thin glue into the split... will see how it holds together. I may have to bring out the PU glue if the crack opens up any more. Bummer.