I am trying to determine if a permanent internet connection is needed to develop an app with Apple's Enterprise Program.
I know that XCode has to be configured with the dev account ID in order to retrieve certificates (thus needing an internet connection), but once this step has been completed, is the internet connection still mandatory?
You don't need a permanent internet connection. It requires when you add your developer account id in the Xcode Preference->Accounts. After that you can download certificates from there. Later on internet connection won't required.
It again requires connection when you are signing the build.