I don't personally know of anything but I think if you were to use something like this:
and attach ducting to it with aluminum tape or something,
you could keep it outside of the area without risking humidity damage. I don't know that you're going to be able to find a heater that can handle 90% humidity long term. Having it outside of the area and forcing heated air in like this might be easier and cheaper. I'm not sure how you would regulate the temperature this way though.