I don't teach aikido. I do, however, coach a sport that is competed at the world and Olympic level. One athlete I introduced to the sport and coached for the first 15 months of her career passed my athletic achievements 23 months after she started rowing by winning the Pan American Games. 4 years later it was two world championship gold medals, the next year it was two Olympic gold medals. Later she retired with (let me see... 3 Olympic gold and one bronze. 3 world gold, two silver. In recent discussions with others who remember, I learn that they think she had the best "form" of any female in the sport. At best I was at "club" level in the sport. I've helped a number of others learn enough of the sport to make the leap to elite level competition, despite my never having coached at that level myself.
Others I have coached either as athletes or coaches have raced or are coaching at elite or near-elite levels. I guess I can consider that a success, even though my "students" passed me by.
Is it ethical for a shodan to recommend another shodan to higher ranking? I think so - if the former can tell that his/her student has passed him/her by and is going on to bigger and better things. Is it ethical for the person to hold the student back? Not on your life.
Forgive me for not having eloquent phrasing