Awesome idea...Before I answered, I was sitting here trying to think of how the batteries could be accessed with a charger from underneath and never even thought of the starter power stud.http://www.revbase.com/BBBMotor/Wd/DownloadPdf?id=878488
Connect your charger (+) to the battery wire at the starter and (-) to ground charge it up and unlock it. It will probably have enough to unlock as soon as you turn on the charger... (BTDT)
Should work fine on a 6.0L. I know it will on a 7.3L, as the solenoid is mounted on the starter with a hot lead connected to it. There's a second somewhat smaller lead that activates the solenoid coming from the fender mounted relay on '99-'01 truck, and from the Relay Panel starter relay on '02-03 7.3L's and is the same configuration on the '03 6.0L's for sure, and likely the same on later 6.0L's.FWIW, going to the starter lead won't work on older Fords that use a starter solenoid. That's why I included the drawing link.
That would work on some of the older trucks that were live all the time to the TT charge line but the SD's have been relayed through the ignition since 99 as far as I know. I got to think the other brands at some point all did the same. I remember coming out to a dead chevy a couple times in the 90's from forgetting to unplug the trailer overnight and something was left on in the trailer.I back fed power thru the trailer terminal on a Dodge to get it unlocked. Using the starter lead idea won't require buying a trailer plug either!