I also really would like multi-currency support (which is also partially discussed and voted for here and here).
After thinking about @Curve_Marie 's point regarding regulatory considerations, I realized that a simple pass-through might really not be feasible. However, a feasible compromise between that, and annoying manual currency switching, might be as follows:
Provide multiple-choice currency selection for every card, with one currency still marked default for transactions not covered by the multiple-choice selection.