This would be really helpful. It gives the chance for a clan to Really see how a player does in say a match compared to a 1v1 tryout on just a simple map or two. The clan would have a week to decide whether or not they want this person in their clan.
As for that issue, maybe the trial would be set at a week. So if someone joins, they join for a week, no leaving. And maybe after the week is over, they must be accepted again by a leader to join the clan. This makes it so the leader doesn't have to be on at the exact week ending to stop the player from automatically joining the clan, if they didn't get a chance to say no.
If the "clan contracts" are still being looked into, they would start after the trial. And there can be 2 options to let a player in a clan, Accept into clan, and Accept as trial. That way people can still join without a trial, if the clan wishes so.