Into the crawl space from the house, you will be creating a negative air pressure in the house as compared to the crawl space. Air from the crawl space will circulate back up into the house. You don't want this for reasons you stated.
I would go with the heat tape. As long as the pipes can be wrapped without the tape comming in contact with any combustible materials, you should be fine. I have personally used heat tape on plastic pipe running on top of the ground at temperatures as low as 25 below. As long as it is properly installed, you should have few problems.
Another possibility would be to install a loop that would circulate water from your water heater using a pump designed for this type of circulation. It could be installed with a thermostat that would turn it on only when the water temp dropped below a certain level. The water flow rate could be restricted to keep the water from getting hot.