This app exists, and for every camera/lens combination
http://www.artistsviewfinder.com/ There is a stripped version too in the App Store.
The iPhone itself is enough for a 55º horizontal lens and down. You can use it with a WA converter when you need more (up to 100º). It's cumbersome, I agree, but sometimes less than any of the alternatives (my Linhof viewfinder goes only up to 62º)
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