Never heard of any such thing, besides that would majorly dilute the prizepool given most usual freerolls
from the sites themselves only run 25 - 100$ or so, and if a site like ftp, for example, that has 3k ppl in a 100$ freeroll, kinda hard to do it as a bounty tourny (given the bounty in any bounty game is an amount as part of the registration, so each player entering has a bounty). Only way it would seem to make sense was if the $$ of the freeroll was determined based on the number of entrants, perhaps maybe like a 1k with a 1k entrant cap and so each player would have a 50c bounty and then 500 towards the prizes (or more if didn't hit the cap to take up half the 1k total from entrants). Clear as mud, per my thoughts on it.
Seems would be doable, but would have to be a fair size freeroll prize pool as mentioned, and have a cap on the # of players to do a proper devide 50/50.