Chad
By Persistent do you mean Cookie on Users Browser OR also on Users Drive.
You could set a HTTP_COOKIE (on User's Browser) response using siteminder responses as part of successful authentication event (OnAuthAccept). As long as the User's browser is open, even if he logs out from SiteMinder (SiteMinder session cookie are killed); the Response Cookie would be present. However if he closes the browser, then I guess that cookie is lost.
You can name this cookie and set any value you'd like (either Static or UserAttribute from UserStore).
Remember this Cookie would be a persistent cookie which is set using responses. Hence make sure you are not exposing any identity threats by setting values that could compromise. There are also way / tricks to reset the Cookie Value (e.g. just before logout OR on accessing a particular resource protected by SiteMinder). But these are solution designs and care should be taken whilst doing these custom solutions.
Hubert