I am exceptionally late to the party but I am looking for help on something else and came across your post.
We do this a lot. For policy reasons we could not have our GTMs participate in DNS for our highest level domains. However we have delegated a sub domain to the GTMs just fine and CNAME specific entries.
example.com is served by our external DNS servers
lb.example.com is served by our GTMs and sub domain is delegated in example.com to a self IP on our GTMs
On the GTMs we setup the basics for the lb.example.com subdomain, SOA, NS etc and then add pools, wideips and so on.
Then we configure a wideip for www.lb.example.com and make www.example.com a CNAME for www.lb.example.com.
Voila, the GTMs are not directly involved in serving example.com.