Could you not contact the mirafloreslock, the ship might have to book in advanced to go through? I suggest a picnic basket, some comfortable chairs..could be a long wait.
I think you should check your dates. Mercury is in Acapulco on the 15th Dec. It goes through the Canal on Wednesday 10th and, given that it is travelling from the Caribbean to the Pacific, it should be at Miraflores between about 2.30 to 3.30 pm as cruise ships have allocated times for the transit. This assumes they commence the transit at Gatun at around 8.00 am. Ask them to phone or sms you when they start. A mobile phone is also useful as you can talk to them as they pass through (just beware of the call cost!).