Old mortise lock bolted from inside
Modern mortise locksets for bathrooms usually have a deadbolt release on the outside, opposite the deadbolt knob (or thumbturn) in the inside. The release is intended to be operated using a coin or screwdriver. 

Your lockset presumably lacks this external release feature. However it is likely that the lock body itself has openings on both sides for the deadbolt. So, in the worst case, you could remove the external lever and cover-plate and then cut a hole through the wood of the door at the position of the deadbolt knob to gain access to the deadbolt shaft.