Might it be worth transferring high-interest credit card debt to a 0% balance transfer APR card and then transferring the outstanding balance at the end of the promo period to another 0% balance transfer APR card?

I do not recommend this tactic. While someone might be able to do this a few times, it will eventually catch up with them and have a negative impact. All credit-related actions are captured and documented on one's credit report, and this juggling gives the appearance that the customer is at increased risk of default. This will trigger being charged an increased interest rate. So, future credit providers will see this pattern and eventually deny the customer's application for the balance transfer. Until a customer's application is denied, they should be expected to pay higher balance transfer fees because of their increased default risk.