I guess I don't understand your post David. It seems to me, that if you drill a hole further down than the bell, as the tank fills it would go out this hole. Even if your filling faster than it's going out the hole, your not really mantaining a full change over, but more of constant flow.
Which is more along the lines of what I have now.

Maybe better to ditch the bell setup and go back to toilet flappers. I can adjust those a lot more simply.