It seems there's an obvious and easy (at least for us taxpayers) answer the US Government is missing. Given that the tax return requires that we enter how much of the 1099-DIV amount is from US stuff, put a new box on the 1099-DIV that reports that amount. Then it's a simple matter of copying the amount found in Box 17 (or whatever). Make the brokerage firm who has all the info in the first place do the math.
