I think it is a great idea! Here is a link to what mine looks like. Convert Wood/Coal Furnace to Natural Gas Hydro-Air
Scroll down the post to see the picture of the homemade humidifier.
Although since you are firing wood and coal, I would option just for a on/off valve instead of using the float valve like pictured and fill it up twice a day. The higher heat will make the valve fail or at least it did on two of the valves I used. The valve even with my gas converted Clayton has failed again and will fail open. Not fun to come home to water dripping out of the bottom of the furnace and on the floor.
As for its performance, I was able to run at a lower temperature and the house feels much warmer. Sometimes, I'll even put some cinnamon sticks in the water to make the house smell good.
Overall a great project that you will be happy with!