Can anyone site any laws that require a standard employer with a standard HMO plan to offer an annual open enrollment period? Furthurmore - is there such a requirement(besides section 125 plans)?

I do not think there is a federal statute that requires an annual open enrollment. For example, HIPAA states you do not have to have an annual open enrollment period. Of course HIPAA does require special enrollment periods. Having just said that, I do believe state laws may require an annual enrollment periods. For example, in Ohio you can not keep someone out of a plan forever.