Yeah, there is a treatment and a outline of the whole trilogy. You don't involve characters requiring specific actors with out their consent. We know Mark and Carrie were told about it already. That they will be involved is merely reading between the lines. Carrie Fisher probably thinks that is enough for now.
I suspect we will hear some more official on Carrie and Mark's involvement soon. The first two of a series of interviews with Lucas and Kennedy are being released on starwars.com. its clear both those are part of a long interview done on a single day. Its possible later segments will have more big reveals.
One thing is clear they are deliberately teasing us all by slowly disclosing information. I willing to bet Harrison Ford has some role too. But that will be the biggest tease of all.