As an update, I just took my runner to SF Toyota and they found the belt problem and fixed it for no charge with no hassles.
They still have a "diagnostic fee" but their terms are more reasonable -- if they can't detect the problem, no charge. If they detect the problem and it's covered under warranty, no charge. If they detect the problem and it's NOT covered under warranty, there's a $135 charge but that is refunded if you then have them fix the problem.
Moreover, I could actually talk intelligently with them about the issue so they could find it.
And the final kicker... City "shitty" Toyota did not check the tensioners, but happily told me that was probably the issue. They did, however, visually inspect the drive belt and said it was fine. SF Toyota took one look at the drive belt and said it was "obvious" that was the problem, and they changed it (after checking there were no relevent TSBs). Problem solved.
Needless the say I'll be contacting Shitty Toyota to demand a refund on their miserably inaccurate diagnosis.