Problem with geolocation with GTM in a delegated domain scenario

Our GTMs operate as an authoritative subdomain, so the only traffic that gets to GTM comes from our regular DNS servers. Because of this, the client source IP is lost and we are not able to take any actions based on the client's IP address. The only workaround I have come up with would be to create another GTM listerner that operates for a totally domain name that's not already authoritative on our regular DNS servers. This would allow the client to query the GTM directly, therefore the client IP would be known.

Have I explained this clearly, and are there any other ways to get the source IP of a GTM client in this setup?