Good idea, Rojo, but you have to extend the deadline. Rossi's purported ankle injury gives him an excuse not to accept any challenge that would have him run between now and the end of the year. Instead, you should extend the 5k challenge deadline into next spring based on his apparent injury. Also, you should keep the 20 minute time limit because, as others have pointed out, running a 5k in under 20 minutes would be easy for anyone who ran a marathon in 3:11 within the past 18 months or so. But, for goodness sakes, Rossi should be required to tell you in advance when and where he is running, or it doesn't count.
As far as having someone shadow him, hell, offer to have someone pace him. Then, have someone else videotape the whole thing. When Rossi sees the pacer take off at a 20 minute 5k pace, Rossi will pull a groin muscle faster than you can say "Barb McKeever."
Here's the deal you should offer Rossi: You offer to pay him $1,000 if he breaks 20 minutes in a 5k on a legitimate course next spring, but he has to announce on Letsrun when and where he will run well enough in advance that anyone who wants to join in the race will have time to register. You further offer that if he runs the entire race, fails to break 20 minutes, and admits afterward that he had no excuse for failing to break 20 minutes other than he is not currently able to do so (which to all runners would be tantamount to admitting that he cheated at the 2014 Via Lehigh Valley Marathon), then you will donate the $1,000 to his father's charity event. Can you imagine how many people would sign up for the 5k race for the chance to "Run with Rossi?" You should still offer him a pacer and videographer. In particular, the videographer would be for Rossi's protection and would ensure that no one does anything stupid (I'm looking at you, Stillman!).
It's a win-win-win. Rossi gets $1,000 for his father's charity event, whichever 5k race Rossi chooses to run in gets a big boost in registrations, and the running community gets something positive out of this whole trainwreck: a ginormous 5k party with the best damn Philly DJ anyone has ever seen. Maybe Rossi's DJ company could host a post-race dance party. I can see it now. 2000 people all doing the chicken dance with Rossi after the awards ceremony. Might even break the Guinness Book of World Records for largest group of people doing the chicken dance!
Rojo, you need to do this, and you need to publicize the proposal on Letsrun, in Runners World, and in the local Philly media. Maybe Cecily Tynan will mention it to her followers. If Rossi turns down the challenge, and the opportunity to raise money for multiple charities that goes along with it, there could be no better proof as to what kind of person he truly is.