I really like this idea! As it currently stands, the ‘transmog’ roll option is basically never going to give you the item, so it’d be nice if it increased your chance of getting it.
For OP’s idea, it would only work if the currency you got for passing was greater/equal to the amount it would cost to buy the replica (otherwise why not just need?), and this would lead to people getting all of the appearances from their first or second run of the raid.
Exactly this, instead of punishing people with better gear already they should try to make some changes to lfr in general. Something to make it actually more attractive for people to queue up more.
I would suggest putting BoE replicas into the goody bag tanks and healers sometimes for queueing up because there is a shortages for those roles. People who want the tmogs would have the option to queue up as roles wich there is a shortage of or buy it off the ah.
I think this would be the best way, in lfr you sometimes have to wait like an hour sometimes to replace a tank (wich doesnt instantly leave after joining). This would give more incentive for people to join for those roles. Wether they want to keep the tmogs or sell them for possibly a nice extra amount of gold