Interesting question!
Ultralights can be registered with the EAA's vehicle registration program, which issues a registration ID beginning with the letter "E". The problem is that in Canada you need a license to operate the aircraft. That is why there is no reciprocation between the US and Canada for that special exemption rule. In Canada, ultralight pilots are required to have a license. If you have a US pilot's license from Recreational on up, and an "E" registered ultralight, you are compliant with Canada's requirements. From Transport Canada's website:
Foreign licence validation certificate (for recreational pilots)
Visitors to Canada may have a foreign pilot licence validated for
private recreational purposes. The foreign pilot licence must be
valid:
- under the law of the issuing state
- and for privileges appropriate to the reason you’re flying
You may apply for a foreign licence validation certificate before
arriving in Canada. The certificate will be issued for a period of one
year. After a year you may renew it or apply for a permanent Canadian
pilot licence.
To learn more about the certificate, including how to apply, read
Advisory Circular 400-003: Foreign Licence Validation
Certificate.
To my knowledge, no other country has the same type of ultralight aircraft rules as the US.