It's probably worth pointing out that the requirement to register with US Treasury also applies to those outside the US who do business as a money transmitter in the US.
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This provision doesn't require that you act as a money transmitter on a regular basis or that you be an organised/licensed business concern. It has a lot of potential to be interpreted very broadly.
You're absolutely right, if you provide "Exchanger" services in the US you would come under FinCEN jurisdiction. Also, right that this rule could be and has been very broadly interpreted.