You wont be able to re-create the Manage Subnets & IPS page, as this is simply not a resource that is available for adding to a new page.
You are right, the best method to provide subnet information on such a shared (NPM and IPAM) data on a page is to utilise the Custom Query or Custom Table resource. You could make it dynamic with some more advanced SQL, where you can do a distinct lookup for the IP address and include matching unique subnets from IPAM that match,
Gah, this is still bothering me. I don't like having to go to 2 different places for information on a single location. I'm taking a stab at a custom SQL query but, it's looking like a pretty daunting task.