I was in your shoes few years ago. What I did was I informed USCIS coordinator in my school saying that I have no job at the momemt but have been volunteering for a professor (though I was not volunteering). I gave the name of my  undergraduate research advisor. I did not even inform my research advisor. After couple of days, the USCIS coordinator replied me saying that he informed the USCIS about the situation and it is under control now.
Things were definitely tougher during that time and it is hard to find the job within 90 days. Hence I heard that USCIS was not strict in this kind of matter. I am not sure about this kind of "halla' these days but I have not heard of anybody being sacked because of 90+ days unemployment on OPT.
Goodluck.