if it were an official Readercon event in a programming room, would con staff allow guests of the KS donor who are not Readercon members?

Very easily, if Rose performs it on Thursday night of Readercon 2015: unless they change the policy between now and next year, Thursday night programming is free and open to the public. If for whatever complicated reasons of scheduling it has to be another day of the con, I like to think that with more than a year's notice we could work out something about day passes with the Readercon program chair.