Humidifier in Return duct
Humidifiers are made to be installed on the supply ductwork, though I have installed them on the return duct successfully in the past when plenum space has been a problem. There will be reduced capacity, but it will still provide you with some humidification. This is because of the way the humidifiers are built. They are using the heat from the supply duct to warm the water in the evaporator pan, or blow the hot air through a wet foam pad, or by some water wick fill sheets, or whatever other method they may be using depending on manufacturer and price. The humidity shouldn't cause any trouble with your ductwork or electronic air cleaner, providing you don't have leaks allowing the water into the return air duct and the pan isn't overflowing or the like.
Your furnace may have a humidifier power supply off of the control board if it is a newer model, or you can power it off of your W and C terminals in your furnace, providing it doesn't draw too much current. You may want to check this incase you overload your furnace 24V transformer and cause it to burn out.
Hope that helps,