I have spoken both with the events and meetings folks and the private dinining folks, and the only available venue on-site at the Rio would be a private room at VooDoo Steak. Creation has booked out all the meeting space in the hotel. VooDoo Steak entails a food and beverage minimum of "$85++ per person." I have an can post the menus, but I'm assuming that's too expensive and at risk of excluding people based on the cost? They require payment upfront in advance of the event.
Other options would be trying to find a venue off-site. @dave knows Vegas pretty well and perhaps could make some recommendations for that. I could also look into event spaces at the Gold Coast next door, but I expect the story will be similar, though probably a bit less expensive. That's where I stay, because it's a lot less expensive.
Another option might be renting a suite upstairs in the Rio. One that could accommodate a reception of this size is $560 for Saturday night. I'd be happy to split this cost with one or two benefactors, but there may be other logistical concerns such as attendees needing a room key to get access to the floor by elevator. The advantage is we could buy and bring our own refreshments.