I would happily pay money for this app, a true shame that it has been rejected. This story makes me wish that there were easy to use sideloading opportunities for app developers within iOS.
Xcode 7 lets you compile and run apps on a device without a developer account. It would require giving out your source code, but at least it's available.
You could give your code out as a compiled static library or framework if you were concerned about that. Then the source code would just be the trivial code required to link and present the app.
Can it be coded with the users public key from their device thus limiting it to work o my on the device with the matching private key?
IDevices do use public/private keys don't they ;)