Swapping for XXXX is nice for detecting unlocalizable text. I don’t understand what you mean regarding the fallback for that mode though.
In regards to the string length checking modes, I’m skeptical. The combination of non-fixed width fonts and repeating the same character can make noticing “perfect clipping” rather hard. Who can tell that only 31 of the 32 Ms are visible when the 32nd M is perfectly clipped off the screen.
I think a better solution would be interactive UI tools. Perhaps we could add a feature to the widget reflector that highlights/simulates common language size increases in text blocks. 
Also maybe a mode which makes text blocks turn a bright color or flash or something if they clip their text values. A good mode for QA.