Have had this very issue on an A320 tankering fuel 3.5 hours to a destination with very limited, and sometimes no fuel availability - a perfect imprint of the fuel tanks in frost above the main and tip tanks on arrival. With an OAT of +13C on the ground and half empty main tanks, the frost soon dispersed above the mains. To remove the frost from the tip tanks which were of course still full of fuel at a chilly -15, we manually opened the fuel transfer valves allowing the fuel into the mains, and then having closed the transfer valves, uplifted warm fuel into the tip tanks. The frost cleared within around 10 minutes.
While operations with any type of frost on the upper wing is obviously absolutely no-go, the following study by Airbus makes interesting reading
SmartCockpit - Airline training guides, Aviation, Operations, Safety