It's a bad solution. It shouldn't even be someone's first idea.
You went to the trouble of implementing this type of matchmaking server-side and in the client, when there was a better and (probably) easier solution.
Have the game automatically offer people extra rewards to fill specific role/s when they're in demand.
If I had a guarantee of getting a champion capsule for playing jg or support (and winning), then I'd be down, and so would many other players. It would encourage them to familiarize themselves with roles they're not comfortable playing in a game like league.
This way everyone is happy, and you're not forcing someone to play a role they don't want to.
EDIT: Offering people different kinds of shards and other rewards would also work, as someone has pointed out that champion capsules / blue shards are worthless to veteran accounts.