Experimenting...
Modified my step-down module with a fan and a volt meter. Built an output adapter cable to go to the cigarette lighter. Hacked up a dead power tool in for input power...
Engine off, accessory on, 13.0v output...~1a output.
Engine on, alt disconnected, 13.0v output, ~4a output.
Engine on, hvac fan on, lights on, alt disconnected, 14.4v output, ~9.5a out.
The latter example is the usual state I drive to work. Car battery read 11.92v, so the 9.5a was being bolstered by the car's battery. After 5 minutes of testing, the cigarette lighter plug was warm. Not great, but not terrible.
So, with that being fairly successful, it was time to go modular...
Made adapters for some of my other batteries, set everything up with EC5 plugs (under $1 a pair on ebay, good for plenty of amps). I can now step up or step down voltage with one or the other converters, depending on my needs, and have an array of different output connections. No more of each battery only serving one purpose.
I've got plenty of watts on tap now, so I can start experimenting with driving without the alternator connected.