Is it permissible for a Muslim living in Canada or the U.S. to sign up to be an organ donor?
If the life of a Muslim depended on it then it is permissible, otherwise, it is not permissible based on obligatory precaution.
NOTE: Signing up [ahead of time] to become an organ donor is problematic because it is not known who the recipient will be. Moreover, organ donation programs in the West do not account for Islamic rules and restrictions in the process.