Hi David,
I am working on an offline app for on premise hosted Mendix application too. Just download the phonegap.zip by choosing build yourself in the cloud portal.
You will have to customize the url in the json config file.
Make sure the on premise app server can be reached from the internet. Also make sure it has an officially signed SSL certificate issued by a recognized certificate authority. I am no network admin so I have no more specific details about this.
Also research the app deployment options: Public appstore, company private, bulk sale of the app.