Yes, I agree about the “on the hoof” nature of diversions and the need for speed.

I would guess the corporate card idea is a must. Also, as a congtingency, why not store a suitable sum of cash (say USD 10,000) in the flight deck?

I am fairly certain that the RAF give their pilots cash in a similar way (though it is in gold coinage or Kruger Rands or something like that).