I've now got some real-world experience to report.
While replacing the high-beams on my wife's car today, I noticed that her battery was marginal and, especially since I'd already invested hours into getting all the plastic covers out of the way, decided to put in a new AGM battery, too. With no battery in the vehicle, I tested my various jump packs and backup storage batteries to see which could start her 3.5L V6 solely on their own power.
DieHard 18ah jump pack, 22lbs for the entire pack, and 12lbs for the SLA battery alone: No start with either unit by itself. But with both units hooked up together it did start easily. Battery level showed a drop down to 75% capacity when I put them on the charger afterwards.
35ah SLA wheelchair battery, 23lbs: No start. Battery level dropped to 50% capacity.
150ah AGM deep-cycle, 97lbs: Easy start. Battery registered 100% capacity.
Noco Lithium Genius Boost, 2.6lbs: Easy start, with just a faint hint of initial hiccup. The pack still showed 100% capacity afterwards.