Pointing domain back to old server during development

In a WPMU multi-site setup, have a client’s site finished and want to point their DNS to my servers. Problem is, they have a live site already at another host. I want to do it with no down time, of course, since they get traffic.

I’ve seen hosts before who did something where I was able to change the DNS on the domain to the new server, but the host pointed it back to the old host until I was ready. Once I was ready, they’d do something and the switch would be instant.

Is this possible within my WPMU admin, and, if so, can you tell me how? I’m hosed at pSek.

Thank you.