Exactly that happened to me (767 Capt) at my last airline. We got the normal cabin secure, commenced the approach, then got a call to say that a pax had unstrapped and locked themselves in the loo.
Fuel and weather considerations dictated that we land anyway but to be perfectly honest I would have landed in any event because of the possible security implications and, at the end of the day, if the pax has a real problem he/she is better off on the ground.
I submitted a Voyage Report and never heard anything back from it. I think there is a case for locking the loos as part of the cabin secure procedure as some airlines obviously do. Thoughts anyone?