I have not delved into the statistical approach for handicapping the Belmont as much as I would have like. That is mostly due to the fact that data is harder to come by. The Derby and even the Breeders Cup are a lot easier to study and observe trends. But I will note a few things.
Now that Rags to Riches is in the race it looks like game on. She will have a difficult task against the boys. In the last 20 years only 4 fillies have tried the Belmont. Genuine Risk finished second in 1980, she came into the race off a Derby win and a second place finish in the Preakness. Winning Colors tried to mimic the feat after winning the Derby and placing third at Pimlico but she faded badly in the Belmont and was beaten by 41 lengths. The next filly to try the Belmont was My Flag in 1996. She was built to be a stayer and was coming into the race directly from the Oaks where she ran 5th as the favorite. The most recent filly to try the Belmont was Silverbulletday. She hooked up early with Charismatic and was run into the ground. So half the fillies did well and the other half did poorly.
In general here are a few factors that I'd want to see from my Belmont selection.
Minimum of 3 starts in the current season.
Must have run at least 3 route races in the current season.
Must have race at 9f or beyond at least once.
Must be a stakes winner.
Must have achieved their highest lifetime BSF in of their last two starts.
8 of the last 14 winners met all of these criteria. The problem with these criteria is that they aren't going to eliminate many contenders. For instance in this years running it leaves Curlin, Hard Spun, Rags to Riches and Tiago which not coincidentally will be the top 4 choices.
Another thing you might want to note though is the relatively superb success of seemingly inconsistent horses. Over the last 14 runnings the average number career starts by Belmont winners prior to the race is 9, the average number of wins is just 3. A lot of "ordinary" horses do very well in the Belmont so don't be too scared to take a shot. I think what happens is that these horses are not really fast enough to excel at shorter distances, but in the Belmont they relish the distance and their competitors don't.
The average odds of a Belmont winner in the last 14 runnings is 14/1 and favorites have a cumulative record of 14-4-4-2 which is just about in line with the average of favorites everywhere.