I think you should seriously consider doing this. It's the best way to measure the true impact of YC on the companies it selects. Otherwise, it's hard to know to what extent you're just picking the most successful teams vs. genuinely making the teams more likely to succeed.

Addition: To avoid having to completely reject people, you could take randomly select half to enter immediately, and let the other half enter in one year. Then you could measure the difference between the two groups after a year.