I've tried it and it's quite impossible tbh. Tried first off by using a blow torch to heat an exhaust but it needed so much heat it was too heavy. The other option is an electric heating element but that also doubles the weight of the planes batteries. Petrol planes give out 70% of their energy as heat whereas electric is much more efficient so it gets difficult. I have seen battery powered 12v smoke machines but they weigh 5 or 6kg.
You could also try pumping the diesel through a very small hole and hopefully when it comes out the other side it expands and vaporises, haven't tried that.
What about an ultrasonic water fogger? you'd have to carry the weight of the water, but the US fogger should be ok on the power. The atomised water looks like steam, but is cold.
No idea if it'd work in an airstream, or if you could produce enough to even to be visible.
btw, great vid! That guys has some great looking places to fly! Good pilot too - flying from a hot air balloon, that's just crazy! Late in the vid I thought, good to see that he dorks it too, then I laughed hard when he fell in!