In the league I'm currently in, the player at the top has placed his highest rated players in the starting 15, regardless of position, there must be a penalty for playing a fullback at centre and a flanker at lock, although this isn't reflected.
There is an element of realism to this persons request. You cannot play an untrained person in the front row in real games, therefore stick a full back in as a prop is ridiculous. Scrum half as a second row?!? Etc
I don't have an issue with players crossing over, so flanker at wing or centre etc, but there should be some sort of negative to this...