He does want the challenge to be slightly possible
Indeed. We're already talking about 398,131,200 forestry backpacks here, let alone filling them with cobble. Filling each backpack with 15 stacks of Octuple(~43M) Cobble.
Sure, it's a lot easier than filling a DSU with Octuple compressed cobble, but (EDIT: NVM, i wasn't paying attention it's filling over 2 dsu) building all those things to house it would be astounding.
Worst part is, if i wanted to store as much cobble as possible, then i'd be putting those dollies into DSU instead of barrels; I'd be using Resonant strongboxes instead of normal strongboxes; I'd use Woven backpacks instead of the basic ones. I basically just wanted to put tile entities into tile entities and then put those into more. The cobble wast just an excuse to fill them with something.