They've had Helen Slater who was the original Supergirl actress on the show as Supergirl's adopted mom and Dean Cain as her adopted dad. I'd like to see both of them make appearances on the show as themselves and maybe even do some splicing. I know Robin Williams guest starred on Mork and Mindy as himself once, and that was fun to watch. The only thing is whether or not they'd get suspicious of the secret identity of Supergirl and Superman.
Take care. God loves you.