Ability to assign an external id/tag to an email asset

We have an external reporting application, where we are extracting all Marketo activity records each day (including the email activity types, such as "email sent", etc.)  We would like the ability to assign an external id in Marketo to each email asset and have this id/meta data available for extract via one of the Marketo api's.  This would be similar to being able to assign a tag to a program -- but at the asset level.

Would be interested to know if any one else has a similar requirement and how they are handling this.