I am certainly not yet an expert with Mega - but I have a few thoughts that may be useful to follow-up with in discussions with your Mega Professional Services implementation contacts:
If you will only ever have one Product Owner for a Product - you could leverage the Property feature in the Social Collaboration - but this would require adherence to a consistent naming convention - and would only be enforced by attention-to-detail. But, it would avoid any need to customize/extend the meta model for your implementation. Or, If you have access to (and are comfortable using) the Admin Tool, you could make a minor meta model change to include a custom field on the Product object
If you will potentially need to support [n] associations of Product Owners, I suspect you may be better served by modifying your Product object in the meta model to include the ability to associate a Responsibility (similar to what is available for Application). - assuming that this is not already provided in the Product object definition in your implementation.
Enterprise Architect, World Traveler, Intrepid Sailor