crn/cognos8 need to store user/pass in the content store at times. for example, when a report is scheduled, the user is authenticated when the report is executed.. and when you schedule, you cant' expect the user to sit there and type a password.
so, in short, no. unless you go about some convoluted way with the sdk and somehow know when the credentials need to be renewed. there is certainly no straightforward way. what makes it even harder is the fact that the user has to first log into cognos to be able to renew credentials. if the user is not already authenticated, you can't renew credentials.