By way of a general update for those who are curious, here's what I learned talking with the local Enterprise rental agent (only car rental option within 30 miles). The collision damage waiver covers only damage to their vehicle. I can pay for two additional layers of protection: liability for damage to third parties ($1M cap), and medical/property loss for me ($2,500 cap). If I bought all three, the cost per day comes to $46. Daily car rental of an economy car is $40 per day pre-tax.
I've not yet found any local agent offering per diem auto insurance, and non-owner liability coverage so far is limited to only 6 month terms, not per diem. I think I'm going to contact a couple of the big bicycle advocacy orgs in the U.S. and see if they have heard about other options.