Hey, I came here looking for an answer, and I have a workaround solution, inspired by a template I saw in the templates folder.
First, make one layout for your large layout, and another for your iphone. Now, make a new persistant symbol in the large layout, and name it
iphoneSymbol. put a text box in it. drag it outside of the layout's area. In the iphone layout, resize the symbol to be the size you want for your iphone. Name your scene something like "scene1." In the large template, make some text, and edit the html so that the id is the same name as your scene (scene1).
Create a new function that you load on Layout load. put this code in your function:
var templateHTML = hypeDocument.getElementById(hypeDocument.currentSceneName()).innerHTML;
hypeDocument.getSymbolInstancesByName('iphoneSymbol').element().children.children.innerHTML = templateHTML;
Now, the layout will load the contents of your div with the text into the symbol on the iphone layout.
Note that I used
.innerHTML because I wanted to preserve the formatting (colors, divs, paragraphs, etc). If you know that you just want the text, you can change that to
.textContent in both functions, but this will give you messy HTML if you have anything other than text in the div.