I plan to go ahead and use the dehydrator, but NESCO's guidelines for jerky put it at the very highest setting of 155F/68C. So, what I'd like to know is: assuming that I am unrestrained by time, how low can I dial in the heat setting without giving microbes a chance to get a foothold?

You can't combine two completely different methods. You can either make room temperature jerky by blowing air across it really fast, which dries it out faster than the microbes can multiply, or you can make hot-air jerky in a dehydrator which kills microbes with heat, but you can't make room temperature jerky in a dehydrator.

If NESCO says to "crank it", and the USDA says that the lowest recommended temperature to make dehydrator or oven jerky without getting food poisoning is 160F, then 160F it is. Or else go buy a box fan and some AC filters.

http://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/meat-preparation/jerky-and-food-safety/ct_index

Illnesses due to Salmonella and E. coli O157:H7 from homemade jerky raise questions about the safety of traditional drying methods for making beef and venison jerky. The USDA Meat and Poultry Hotline's current recommendation for making jerky safely is to heat meat to 160 °F and poultry to 165 °F before the dehydrating process. This step assures that any bacteria present will be destroyed by wet heat. But most dehydrator instructions do not include this step, and a dehydrator may not reach temperatures high enough to heat meat to 160 °F or 165 °F.

So, NESCO adds:
http://www.nesco.com/files/pdf/nah_adj_cu_manual_30001451_update_102011.pdf

After drying, heat it in your oven at a minimum temperature of 165F (74C) for at least 30 minutes as a precaution against the risk of salmonella.

If you're going to use the dehydrator, follow the instructions in the manual and don't try to improvise or cobble together two different methods.

/u/ZootKoomie Really?