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.
(2) Foreign-located money services business. Each foreign-located person doing business, whether or not on a regular basis or as an organized or licensed business concern, in the United States as a money services business shall designate the name and address of a person who resides in the United States and is authorized, and has agreed, to be an agent to accept service of legal process with respect to compliance with this chapter, and shall identify the address of the location within the United States for records pertaining to paragraph (b)(1)(iii) of this section.
http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&sid=532af3285642b30728036e7a8059b718&rgn=div8&view=text&node=31:3.1.6.1.6.3.5.9&idno=31This 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.