It's very hard to do but you can at least make your SDK do a fast app switch to safar rather than use a WebView. Of course evil apps can find a way around it like you said but at least you can make your SDK and Documentation point the right way,

I am not experienced with iOS but I also suspect there are more advanced WebView detection tricks. It also doesn't help that Apple really doesn't like fast app switching.

you email the authn link to the user rather than use openurl