Thanks for that Steve. I had a recollection that there's a way of referencing the PR itself as a branch within the main repo (I guess it must *be* a branch, as how otherwise would github be able to do things like get Travis to build it?) but I don't recall the details.

Hmm, a bit of googling later I found https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch origin pull/ID/head:BRANCHNAME

And indeed that works - I checked out Nick's patch like that. I'd still need to merge it into the 3.6 branch, which is another set of git commands I don't yet know (cherry-pick, maybe?)