No Dice. The thing weighs 600kg. Even Karlach and Lae'Zel, both with 19 strength and used enlarge on, only get 288 capacity.

Edit: But I can 100% confirm that's the issue. I loaded an autosave just before killing the Lava Elemental, and from there I can travel between Act 1 and 2 no problem.
Means I have to rekill the Goblin Leaders and do the entire Mountain Pass again... oh well.