There are a couple of things to consider when trying to make warm mist... It isn't impossible, but there are some challenges.
1) A heater in the reservoir can help, but bacteria can grow This is not a problem if you clean the reservoir and system out with a mild bleach solution every few months.
2) You have to limit the heat to what the pump can tolerate; My AquaZamp pumps are rated at 35*-115*F. You could put the heater after the pump, but then it needs to be able to withstand the pressures the pump is creating. I don't think fish tank heaters are meant to have very much pressure pushed through them.
3) The fittings in the system are pressure rated based on the temperature of fluid flowing through them. The higher the temp the lower the pressure rating.
4) Warm mist is a serious competition with the ambient air temp. We are contending with physics here. Water that is atomized will naturally cool. In order to get warm mist, you need HOT water. Again, you are limited to the hardware's specifications on how hot and how much pressure you can operate.
Personally, I don't use heaters in any fashion. I have my cages set up so my chams can be in or out of the mist if they wish. It doesn't seem to bother my chameleons to have water that is room temp (70-75*F) sprayed on them.
I realize some people want to pursue a heating solution, I can help however I can, but currently am not really working on any solution.