Given a MySQL system with multiple remote users (users of the form \'joecool\'@\'192.168.1.2\'); is there a SQL statement I can use to REQUIRE SSL
You can configure mysqld with require_secure_transport.
[mysqld]
...
ssl-ca = ...
ssl-cert = ...
ssl-key = ...
...
require-secure-transport = ON
This capability supplements per-account SSL requirements, which take precedence. For example, if an account is defined with REQUIRE SSL, enabling require_secure_transport does not make it possible to use the account to connect using a Unix socket file.