Why can't you just join that computer to the domain and be done with it?

One of the main reasons for joining a domain is so that the adminstrator group on the domain has access to its administrative shares.

Are you trying to work it out so that you can have admin privileges on the local box, yet still participate in the domain? That's easy, just add DOMAINNAME\domainuser to the local box's administrators group after you've joined it to the domain. That way the box is part of the domain, and you can log in with your domain account and still have full access to the box itself.