I can see by your question you've taken the time to think about how to leverage group policies in order to grant local admin access rights. I don't see anything wrong with your scenario.
I think what makes this work is the use of the Global Policy Object (GPO). The purpose of GPOs is to get around the fact that various users with similar access requirements may not be in the same Organizational Unit (OU) in the directory. As you stated, once you set up a domain global security group to the GPO, it can then be linked to sites, domains and OUs containing the administrator user objects. The GPO script would then be linked to the local administrator group of the user's computer. It makes perfect sense.
For more information:
- How do group policy objects and the 'Password Never Expires' flag interact? Read more.
- Learn more about using batch files for temporary local admin rights.
This was first published in February 2010