I'd try to avoid it. Acid (FFA) of WVO is not really the issue. Any water in the WVO will cause the copper to oxidize. There is also considerable evidence that copper acts as a catalyst for polyerization of oil (sort of a linoleum like goo).
As a side note, copper tends to become less flexible, even brittle when heated. If you solder an in-tank copper heat exchanger make sure it is very well supported. If it cracks over time, you'll be dumping coolant in your tank.