Thanks for the idea, and I can understand what you're saying.
One way to solve for this would be to create a Lookup Table Extension that outputs the clientID. You call do a check the domain, and output various values based on the domain value. In the 'Default Output' field you would put the Test Client ID. This way if there isn't a match in the Lookup Table (i.e. any new dev sites) then the Test Client ID will get set.
Make sure all production domains are specified in the Lookup Table. However, not all dev domains need to be explicitly set because the default output will pick all of those up when there isn't a match in the table.
Personally I like this method better anyway because if you have a long list of domains, this is easier to manage and visualize.
Then you would map the output variable back to the CoreMetrics tag as the clientID. The ClientID can be found in the Config Tab within the Mapping Toolbox.
Hope this helps.