I don't believe that what you are proposing to do will work. I'm not sure how you were able to do this in Windows XP, I fired up my old XP machine just to see if there was something I was forgetting regarding the process and the behavior of the XP machine is exactly the same as the Windows 7 machine.
In order to participate within a domain structure your machine has to point at the machine that is capable of resolving DNS queries for that domain or at the very least a DNS server that is able to forward DNS queries reqarding that domain to the machine that can resolve them.
The process is done within the Computer name, domain, and workgroup settings in Windows 7, or under the "Computer Name" tab of the computer properties in Windows XP, in either case, it's done within the "Member Of" context, which suggests participating as a member machine in the sense of the Fully Qualified Domain Name convention as in ClientMachineName.DomainName.TopLevelDomain (DNS root) and as a requirement there needs to be a DNS server somewhere with the necessary Service Records and Host Records that provide resolution for that information. And in many cases, like Microsoft Active Directory Domains, additional authentication is required so the DNS server has to have the Host records for the Domain Controller responsible for maintaining a list of domain members.
I'm trying to understand the need, benefit or purpose of having a fully qualified domain host name associated with your computer but not wanting the computer to join, or participate as a member.