Hi Martin,
GeoShield uses the Java Persistence API thus it should be also able to interact with other databases, But it is never tested.
If you have some skills with Java you can try by downloading the source code, otherwise the plug-in for GeoServer could be also an alternative. The plug-in needs just a JSON file that can be created extracting the user/groups authentication and access constraints from your MSSQL database.
If you need some more indications just ask..
Best regards
-- Milan A.