I see no problem with adding a "clear required" checkbox to prevent folks from joining a party if they do not yet have the clear.

If a person believes they have the experience and equipment necessary to succeed in a farm party even if they don't have the win, they can /tell the party leader and ask that an exception be made in their case. The leader may or may not agree, but it's still better than trying to sneak into a party for the win and hope you don't get noticed.

Farm parties are not intended for players looking to get their first win, and allowing tools to prevent this from happening in no way stops players from forming learning parties.

If there's anyone in this thread who makes a habit of joining farm parties for their clears and objects to this for that reason - yer doin' it wrong.