We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "Dave Liquorice"
saying something like:

Vinegar.

I'd already thought of that and any other acid as you don't know how
much to use so could end up with an acidic residue rather than
alkaline.

Idealy the products of reaction with the battery residue need to be
neutral and the substance itself neutral. I know bicarbonate of soda
would be good for acids but I did organic chemistry to A level so the
inorganic side is a bit, erm, hazy...

I'd flush it out with plenty of water afterwards, in any case.