What is the Positive connection in the engine bay directly connected to? That's right the battery. If you connect a battery to the two jump points in the engine bay it is exactly the same as connecting the battery in the trunk.

yep

Maybe I'm missing something on the alt version: if the battery is disconnected in the trunk, how does a power source attached to the batt lugs under the hood, send juice to that batt?

No biggie, as the tailgate unlatch isn't that difficult a move; I've done it, and I'm not 20 sumpin'. ;>)

Yeah I've got off topic sorry and forgotten about the tailgate. All I'm saying is that the circuit is not in series it is in parallel. With the battery disconnected placing a jumper battery or leads from another vehicle through the engine bay will not send juice to the disconnected battery (obviously) but it will send juice to the truck in order to do whatever is needed (i.e, pop the tailgate)

Should work.
Its pretty much a direct connection from the battery ground to the body ground connection up front. Same story with the power lead. That is why you can jump a totally dead battery.

I have done this one...lol