There never does seem to be any pattern to getting your massages - dont know if boarding early does increase your changes - dont think it does - depends on how full UC is and how many poeple want massages. You always have a better chance on West coast flights though as there's obviously more time.
Just explain your situation nicely and I'm sure she will fit you in
I've only been missed once - and that only was bacuase the beauty therapist was ill so wasn't on board on a BOS-LHR flight.
I suggest you sit in the front section of the UC cabin on the 343 as it's quiter - also I recommend you sit in one of the middle 2 seats (rows D or G) as you then dont need to climb over anybody to get out of your seat.
Have a great flight
James