Hi Neil,
Great question. The best way to handle this is with one tag instance that makes use of a lookup table.
- create a new Data Source called "google_account"
- add a new Lookup table extension
- scope to Google Analytics
- Lookup Value In: select "domain"
- Destination: select "google_account"
- keep the other options as the default
- Lookup Match: enter a test domain e.g.
www.tealium.com (no protocol or slashes)
- Output: enter the test Google account number
- move to the Tags tab
- "Edit" the Mapped Data Sources
- choose "google_account" from the drop-down
- click the "Select Destination" button
- when the Mapping Toolbox modal appears, select "Account (Override Default)"
- Save/Apply/Publish all changes
This will run the lookup table each time the Google tag runs and dynamically override the setting in the default tag configurations.