I found that alternator output can be interrupted by unplugging the 4-pin connector. The dash displays a message saying to check the charging system, but the message goes away after the 4-pin connector is plugged back in.
I purchased a used 4-pin connector and will test to see which wires must be disconnected to kill the alternator output. Then I'll use the spare connector to splice in a switch, preserving the original wire harness.
This is what the 4-pin connector looks like.
Below is a spreadsheet showing measured electrical loads of each accessory, and an estimated run time in hours that the car can be driven. Measurements were taken with a digital clamp meter.
My goal is to have 90min of alternator-free run time, so a 40 Ah LiFePO4 battery seems appropriate.