What's really needed is an IOU that gold will be delivered when the original bearer of the IOU is returned in good health.

Having gold on one's person has been done by militaries but considering that the bearer has no negotiating power if the gold has to be used it's a long shot whether it would help. It just makes it more worthwhile to hunt you down because the reward is delivered to the hunter immediately.

Meantime the poor schmuck is lugging around something that's awfully heavy.

Andy McNabb's book BRAVO TWO ZERO about an SAS patrol which was discovered and hunted down in Iraq describes that the men had gold coins in an inner belt for use in case of emergency.
Didn't help them at all. (not that it proves they'll never be worth anything....)