I don't see how the source of heat will change the humidity levels in the air.
I have radiant heat throughout my entire house in the winter, not necessarily by specific choice we just happened to buy an old house that had it. Some new houses are going radiant heat because it can be more comfortable, there tend to be fewer hot and cold spots and heated floors everywhere are a pretty sweet luxury (we don't have that, we have the wall baseboard things).
Anyhow, I have thought about hooking up my aquarium to the boiler. Basically I would put a little heat exchanger in my sump (probably a "wort chiller" since they are cheap and would work well) and then plumb it in as if it were another zone in my system (my house is already split into 3 zones for water flow, so adding a 4th wouldn't change much).
But my reasons would be financial, not comfort. At least where I live (Minnesota) natural gas works out to be about 1/3 the price per unit energy as electricity. So if I'm heating 300 Watts around the clock that works out to be around $330 / year with my electricity rates, but with a 90% efficient boiler that same heating would only be about $100 of natural gas.
I haven't done it yet because all of the little things would make it a fairly expensive / complicated project (actuated valves, zone controller, heat exchanger, plumbing) and I don't plan on being here forever so didn't seem worth it. But if you are in your place for the long haul, and you are looking at more like 2,000 W of heating then yes it would certainly pay to have the area heated by natural gas instead of electricity. However that natural gas could be water recirculation or just a forced air standard heater mounted to the side wall somewhere. To solve the high humidity what you really need is air flow, dehumidification, and tanks sealed as well as possible to limit evaporation (or better yet, nearly perfectly sealed but with bubbles for air turn over and venting the exhaust out of the room).
