Indeed. I have been thinking about that. I've actually been thinking about humidity for about a month on and off, and while talking to LLJ off the forum about it.
One of the things I considered was building my own tank out of wood, such that there was glass at the bottom for the bit under water, and then above it was kind of like a vivarium for a reptile - with solid sides and roof and then some sliding glass doors at the front. This would keep the humidity levels in the top part a lot higher than they could ever be in an open topped tank - even if the water level is dropped significantly below the tank rim. The disadvantage of doing that is that there would be a wooden section across the front of the tank which would obscure the view.
My next thought was possibly having part of the tank covered with glass covers. The disadvantage of that is that there wouldn't be much room between the covering glass and the water surface, unless I got a taller tank, which as I said before knocks the price up to a point where I probably can't afford it.