Schema extension with unique value

I would like to know if it's possible to perform a schema extension to add a new field to active directory and then require that the value entered into the new field be unique between all users.

For example say I want to track computer to user assignment by adding a field to record the computers serial number. I want to make sure that the same computer is not assigned to 2 people so when I enter the serial number I would like AD to make sure it's a unique value between all other users.

If this is possible any links to documentation on how it would be done would be much appreciated.

Thanks for the help