One the exchange rate: I think the size of the coin was exaggeration for effect. Anyway, at one point I did some research on this subject and the exchange rate is actually correct if you go back to the mid 1800s. This makes sense given that the British wizarding world seems to be stuck in that same time period.
As a result the whole monetary system of Britain is actually deflationary. Put another way, if you do a constant exchange analysis, all that gold actually gains value just be existing.
Of course, that's all meaningless. For one, I would assume doing something like that would violate the statute of secrecy, even setting aside any financial regulations that the Ministry of Magic has. Or the fact that defacing money is usually a felony. If a particularly clever muggleborn, especially a young one, came up with the scheme it would be noticed because they don't come into the world with much in the way of liquid assets. And a pureblood with lots of money isn't likely to think of it because it seems the vast majority have no clue where the muggle world is concerned.