That is a great idea Rolf. Unfortunately it wouldn't catch the hidden code. I know because I just tried. I did exactly as you said and dropped it into an open vi. It looked innocous,so I ran it. I was surprised when the second dialog popped up.
Although it shouldn't be too surprising. I remember someone telling awhile ago of making vis with transparent icons and dropping them as subvis....
They'll still show up in your VI hierarchy, they will be found by VI Analyzer, and the block diagram cleanup will find them (including this one). There was a challenge several years ago on how to hide code in LabVIEW and I think I remember finding a few that hid code pretty well. Not that I'm the kind of person that does this for every VI I find on the forums.
I've also seen some code obstructions not work well due to the block diagram clean up.