I believe you are correct about the Martha R. As far as A/C goes I think Doug has a great idea here and I'm very much considering emulating this once I end up with a bunch of dud parts
(it's inevitable after all). Probably quite a few years away.
However - let's put this into perspective. The average cruising party chartering a newish L420 is going to be forking out $10k plus or minus a few for their vacation
all in. If they want A/C I don't imagine a couple of 100 bucks here and there is going to be a big deal. As a Brit living in the US I am staggered by the expense my friends are prepared to pay keeping their houses feeling like fridge in summer. But that's their choice and I respect it in the same way I respect the choices of future charterers of all boats.
The A/C on the 420s has a certain BTU rating which (and please correct me if I'm wrong) is significantly greater than boats to which she has been compared to on this thread.
Some will like this - some won't. Plenty of options to go around.
I personally believe the L420 in charter
will appeal to those who are looking for real comfort. It will also appeal to those who want to be "green" and choose to use the boat that way. And oddly enough, the boat will appeal to those who are 'twixed' and want a bit of both but are not sure of exactly what this means. From a business perspective I am not worried.
As for having a high tolerence for heat and humidity - all I can say is I know I'm not alone. Unless I am grossly mistaken, a good percentage of cruisers have no A/C and enjoying their lifestyle. Charterers will be more fussy and they are being catered for. I'm wearing my cruisers hat when I make that statement.
Good luck with your trip. Hope it works out.