I know you're not supposed to make starters with dry yeast, and I never have. I just rehydrate for 15 min or so then pitch. But since I no chill I can make real wort starters. I cannot think of a reason why I couldn't make a starter with dry yeast in a rws. The way I look at it it's just like repitching which is totally acceptable.
What do you think, sirs?