iOS Scrolling

Well its not Modal Frame that is the problem. iframes in general are not scrollable on recent iOS releases. The way around it involves adding the style "-webkit-overflow-scrolling:touch" for the container div that wraps the iframe. To do so you have to hack Modal Frames parent.js file.

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On about line 142 of parent.js find the following:

self.iframe.$container = $('<div id="modalframe-container"/>').append(self.iframe.$element);

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Replace with:

// My_Mod added following 6 lines and on 7th line inserted '+ mobileScrolling +' before id="modalframe-container"
var device = navigator.userAgent.toLowerCase();
var ios = device.match(/(iphone|ipod|ipad)/);
var mobileScrolling = '';
if (ios) {
    mobileScrolling = ' style="-webkit-overflow-scrolling:touch; overflow:scroll"';
}
self.iframe.$container = $('<div'+ mobileScrolling +' id="modalframe-container"/>').append(self.iframe.$element);

How about trying something like overthrow?

https://github.com/filamentgroup/Overthrow/

It seems to me you could load overthrow.js on the page, and add a CSS class like:

.overthrow-enabled #modalframe-container {
    overflow: auto;
    -webkit-overflow-scrolling: touch;
}

PS: This is not a bug in modalframe, but something that does not work properly on certain browsers.

Approach suggested by @markus_petrux should work! Closing.