By "proxy" I didn't necessarily mean "web proxy". If at any point an IP address is visible it is feasible to exclude transactions that involve US IP addresses. This is, of course, trivial to circumvent - which was my point.
If there is a single Bitcoin mining node (out of thousands[0]) that will accept your transaction, you can play.
Correct. There is absolutely no way whatsoever to determine the originating IP address of a bitcoin transaction, short of global network surveillance.