Evidently, The National Bank of Canada serves as the intermidiary for foreigner nationals trading with Cubans. So, to keep US$ out of Cuban hands, OFAC is leaning on The National Bank of Canada.
But notice that there are no first hand public accounts of OFAC's correspondance with the Nat'l Bank. We hear it all second hand. To control negative publicity, these things are customarily handled behind the scenes.
From the WSJ.com,
Embassies Try EurosAs U.S. Restricts UseOf Dollars in Cuba
ReutersNovember 7, 2006; Page A6
HAVANA -- Stepped-up enforcement of U.S. sanctions on doing business in dollars with Cuba is forcing governments to change how they finance embassies in Havana, diplomats said. The practice of wiring money to U.S. banks and sending U.S. bank checks to be cashed by Havana embassies has become more difficult due to the tightening of rules under the Bush administration, they said.
Foreign governments were sent scrambling in September when state-run Banco Metropolitano, the only Cuban bank that accepts checks from American banks, informed clients its traditional intermediary, the National Bank of Canada, would no longer process checks without an attached U.S. license that was no more than two years old.
When embassies tried to confirm or renew their licenses they found it couldn't be done. Businessmen said wiring dollars into Cuba was impossible as all transfers were eventually cleared through the U.S.