Just few things to be noted as to why you should use standard workflow here and standard ways to release documents:
1. Managers and CEO's have to release many other documents than just Preqs. Using standard SAP methods gives you one stop to release all your documents like PO's, time sheets, trip approvals..
2. To release a Preq, managers now have to search through other emails in outlook. In standard sap, you already have items grouped by sub category in your Business Work Place
3. Its complex to integrate outlook email with SAP transactions than using standard SAP.
4. Tracking or error handling is easy in standard SAP.
5. Escalation logic: If a manager is on vacation, his higher level or sub ordinate should release document if it is not approved in defined business days. It would be difficult to handle these scenario's in method you are stating. Its easy to implement in Workflow using standard sap method.
Hope these points help them to use standard SAP method. If not, as stated above by Dev and Rahul, you can achieve using user exits.
Regards,
Shahu