Wouldn’t bets disappearing from the pools at the off if they then fall below condition have a dramatic affect on the odds of all other horses. Also consider the implmentation challanges of having realtime single source pool numbers. You think players complain about odds changing during the race now, this I believe would be that on steroids. Maybe something like this is possible at say the local ADW level which is already in place to some extent but it isnt at the off. Kudos for thinking outside the box though.

Australia's TAB has a pari-mutuel Fixed Odds Wagering model that was researched by the late Stan Bergstein and his team several years ago. There's widespread agreement that locked in pricing has tremendous appeal and upside.

I could write the software to do it here. It would make win betting a lot more attractive. In all but the biggest pools where what you see is about what you get (NY/CA/big days), I avoid win betting because at the end of the day, a horse might be a good wager at 5-1 but not 5/2.