Not so, the rudder causes the same roll response no matter which way up the plane! You can, and I have often, fly a 3 channel model doing inverted circuits and the rudder behaves the same, right rudder means roll right. If the rudder's roll effect reversed at negative AoA then it would also reverse when upright and pushing well forward on the stick. Dihedral effect works the same whether AoA is +ve or -ve.