Yes, I admit that the 5-Star Actors like Ford and Jackson will always say no, but it seems to me that the logistics could be made work out for anyone below that level.
1: Move the autograph locations into the inclement weather location. This allows a more comfortable atmosphere for the celebrities, with air conditioning and a regulated flow of autograph seekers, similar to a convention.
2: Offer a healthy dose of money. On the convention circuit, $20 for an autograph seems like the maximum amount that people would not rage over for non-5* actors. If my numbers are correct, each celebrity has 60 autographs for 3 sessions, 3 days in the weekend, altogether totaling 540 non-standby autographs, or a relative value of up to $10,800. Additionally, offer to make a tax deductable charitable donation of a certain amount in that celebrity's name for every "x' standby autographs they fulfill
3: Negotiate that price downward by treating said celebrities like gods. Throw ever last minutiae of WDW hospitality at them, giving nothing but the absolute best.
4: If all else fails, play a little dirty. The fans are these celebrities' lifeblood, allow the public to have full knowledge of how much money their heroes would turn up their nose at to avoid thanking the fans. See what that does to their pay for autograph lines at the conventions.