I see your point for efficiency, but if you did something like yourdomain.* and you don't own every extension (ie. .fr .it .nu, etc.) then when someone else buys
yourdomain.cr that you don't own yet, they want to register their single domain with CAPCTHA, what would happen? Does Google issues 2 separate API keys for you and them? Should both keys would work for
yourdomain.cr?
I think if you can take the time to purchase another domain, you have the time to add it to the list of CAPTCHA API domains you have registered on your account.
I could be wrong though. Maybe Google has a way around this?