That's a great question!
There really isn't anything standard at this point. I have used transcriber-added rests matching the beat value of what's missing. That helps with the counting of the measure - knowing how many beats are missing and being able to properly evaluate what is already there.
The word sign O could work too, if there is no ambiguity with the counting of the measure. You could try out both ways and see which one feels better. (Be sure to give the student a heads up in a TN whichever way you chose.)