If you contact the special events coordinator (used to be Ms. "Enid"), a sweeper will be scheduled for a prior-event sweeping.
Getting a "contract" sweeper might not be permissable on the Stadium lot since it's controlled by the city. They have the resources and manpower to handle the sweeping (as well as paving, port-o-pot rentals, etc) and have been known to block "outside" companies for certain services on occasion (like paving repair

).
I'd recommend contacting Enid (if she's still the person to call) prior to having a contractor doing it. You may have to pay the contractor a full fee and still not get the sweeping done if the stadium "blocks" the contractor

.
Note of History: I used to call Enid on the Monday before an event when I was AX co-chair. She usually had the lot swept by Thursday, or Friday by her staff. I had about 95% cooporation from them. It wasn't perfect, but it was a heck of a lot cleaner than it's been the last couple of years.
Good Luck!
Dan Chambers"It's
just a "well prepared" street car ... or a very, very well-mannered track car."

1983 SC #91 3.6L, "Black Pearl" Livery
1987 944 (gone but not forgotten)