Follow-Up:
Official word from Winnebago would be "no", but they do say that their roof can support a 200lb person for maintenance purposes. I have seen LeSharos with roof racks on them, and have heard of Rialtas having roof racks for canoes and kayaks as well. Winnebago Customer Relations was helpful in providing directions on where to find good attachment points to these people installing the roof racks. I guess the problem would also be how to get the kayaks up there, as Rialtas don't have ladders either. Anyway, I'm sure it can be done, but I would check with Winnie for where to attach it. You may also want to post your question to the Rialta or Rialta-Tech Yahoo Groups because that is where I heard of other Rialta owners having a roof rack installed. You can get to the Rialta groups from the link below -- just go to "Message Boards" from one of the menu bars of that page.
Here is a link that might be useful: MotorHomepage Rialta Resource