I went with a church group, and we hand carried all sorts of things that are difficult to get in Cuba, such as over-the-counter medications.
One of the places we visited was the main synagogue in Havana, and they told us that they receive donations from U.S. Jewish congregations via Canada or Latin America. In other words, the American congregation mails prayer books or other religious supplies to a congregation in Canada or Latin America and has them forward the package.
If you have a trusted friend in some other country, have them forward the package for you.
It's ridiculous, I know.