UA offers the same type of system where you purchase a certain number of miles. You automatically get PremExec (I think).
A friend did it before she had her baby--price per mile was more than restricted tkts but less than unrestricted tkts--she investigated each flight she took during the year and used it when it saved $$ but skipped it when she didn't.
I can't remember what the program is called, but they have a banner at the Px/1K counters at O'Hare.
JL