If I remember correctly the IJN plan was to target just the Panama Canal lock gates not the whole canal, disabling or destroying the lock gates would have caused serious problem for the USN for at least 6 months to a year in moving resources around from the Atlantic to the Pacific and visa versa. If it wouldn't have been for the IJN reversal of fortunes at Coral Sea and Midway the attack on the Panama Canal lock gates probably would have got the go ahead. And if I recall correctly right before WW2 broke out one of the US armed forces in a war game (don't recall if it was USN or USAAC) sucessfully destroyed the locks in a simulated suprise attack, so in theory it could be done and seeing how the higher ups in the US military at the time generally played down or down right ignore lessons delt out by mavericks like Billy Mitchell I wouldn't be suprised if the mission did take place that it probably would have succeeded. I also remember a TV interview with one of the IJN pilots who would have flown that mission had it happened and his words were that he and the other pilots had no intention to bomb the lock gates, but instead intended to crash into them kamikazi style.
"It's the man, not the machine" Gen Chuck Yeager