I'm known for wild ideas, especially when I have a good feeling I could pull it off. However, you'll be wasting a lot of energy, for the insurance that pipes won't freeze, and as another member stated, probably hasn't happened in the past.
If I were to try the "air" method, I'd probably try to run an attached, insulated horizontal duct into the crawlspace, over to the pipe area, somehow encase some of the pipes within the ducts, and then, return that air back into the living space. The booster fan could be mounted horizontally in the crawlspace, and set on a timer, a basic air filter..... geez...don't do it this way....
I think you nailed it with the electric heat tape. I'd use the pipe tape. It is much simpler and has a thermostat in the plug. Try to prioritize any pipe that is 2-3ft from an outside wall. Secure it in place with tape, cover it with pipe insulation (per mfg recommendation). You could go all fancy and get the foam 90° or TEE shapes, but those pieces just add cost and would not be required...you'll figure out how to get maximum coverage. It is also OK to go up 1/4" on pipe insulation to make space for the heat tape.
Keep in mind, you don't have to have heat tape along the whole length of pipe to be effective as heat will transfer from the metal and water in the pipe. I would probably insulate most all of the pipe. I'll bet you would be surprised at how warm the crawl space is on a typical 0F° night. However, if you have several days of -10F nights and the house is empty, the pipes could freeze, or worse.
(With the heat tape, you could (should) use an extension cord to make sure the plug is under crawl space. That way, if it is 25F° outside, but the crawlspace temp is still 45F, the heat tape will now consume energy....
Good luck.
