The name I used is indeed the correct string to make RadiusClient.getAuthProtocol(String) return the correct authenticator. Relevant source ref from RadiusClient:

         // Supported Authentication Protocols
        registerAuthenticator("pap", PAPAuthenticator.class);
        registerAuthenticator("chap", CHAPAuthenticator.class);
        registerAuthenticator("mschapv1", MSCHAPv1Authenticator.class);
        registerAuthenticator("mschapv2", MSCHAPv2Authenticator.class);
        registerAuthenticator("mschap", MSCHAPv2Authenticator.class);
        registerAuthenticator("eap-md5", EAPMD5Authenticator.class);
        registerAuthenticator("eap-mschapv2", EAPMSCHAPv2Authenticator.class);
        try
        {
            registerAuthenticator("eap-tls",  "net.jradius.client.auth.EAPTLSAuthenticator");
            registerAuthenticator("eap-ttls", "net.jradius.client.auth.EAPTTLSAuthenticator");
            registerAuthenticator("peap", "net.jradius.client.auth.PEAPAuthenticator");
            //registerAuthenticator("eap-aka",  "net.jradius.client.auth.EAPAKAAuthenticator");
        }