Interestingly, the chain (and the associated tools required to properly brake and rivet a chain back together) and sprockets as well as a spare battery are the 2 items that are causing me the most angst - heavy to carry but both could fail unexpectedly (especially being bounced around in the Aussie outback).

I could get around the battery scenario by buying 2 Shorai LiFe batteries (very light and small) and wiring them in parallel with a switch (dual battery setup similar to the way most outback 4wds are wired up here in Aus). The chain and sprockets could end up as one of those "one unlucky rock" and your stuck scenarios, so I'd probably get spare Al sprockets and a new chain (sacrifice one can of beer to make up for the weight!), relatively light except for the chain.