Southwest does not have "transcons" per se--if you mean coast-to-coast flights (thus the term
transcontinental--across the continent). The close thing they have to a transcon is a BWI-LAS flight. It is an 5.5 hour flight, but not a transcon. I commute on Southwest a good bit, and I have never heard one of their passengers mention the number of lavs as a factor in flying or not flying Southwest.
P.S. Their flight attendants don't use the lavs as a place to hide from the passengers. They have this odd concept (and corporate policy) that when you are being paid to be a flight attendant, you should be out in the aisle attending to the passengers on that flight. Imagine! No place to spread the latest AA-US merger rumor.
AA has 3 lavs on all its 737s--only one more than WN. I won't say it's wildly convenient for the passengers, but since you can't charge for the "lav seat"--that pesky FAA won't let you seat an oversold there

--I doubt you are going to see ANY airline using up sellable floor space on the a/c with more lavs.