Bottom line:

What do we commonly see other successful programs do with prospective missionaries?

Encourage them not to go, or at least hope they don't.

Rick Majerus was in Sports Illustrated, I believe, taking about how he thought Britton Johnsen may develop into an NBA player if he stayed. He had this conversation with the Johnsens and the dad said something like, "that sounds good now but what about eternity? "

When it comes down to it, and their own self interest is at stake, people know the mission thing is rarely an advantage and has tremendous downside, especially with elite athletes.