Don't see it working. Would tick off diamonds if an FO on a lower fare class got an upgrade over them, and would tick off people that paid and still got stuck with a middle seat in the back of the bus. You would be better of just paying for first class. I would be very pissed if a FO on an X fare paid $75 and trumped me as a diamond who had a B fare (and paid a lot more in $75 fare difference for)

Something I would love to see Delta do (and I know it will never happen) is offer discount to elites at time of purchase to book right into first base on status. If there is a $300 difference, maybe offer 5% discount for silver ($15), 10 for golds ($30), 15 for PMS and 20 to 25% for diamond's. If I was offered a 20 to 25% discount, I would buy more first class seats.