Hi hoolio, hi sparks. Thanks for the ideas. Indeed to manage it through a STATS-profile seems like a good idea (hoping that it doesn't have an impact on the performance, but I'll see that in the tests).
But one thing I do not understand. What do you mean by using the session table to store the count?
Unfortunately the setup is even a bit more complicated (I didn't expect that that might an issue, so I didn't reference it): these 3 percent of the traffic should only affect users who don't have a session-cookie yet. So 3% of the users who don't have a session cookie need to get a different cookie as well (besides being redirected (by 302) to anpother subdomain)
All other users should be left untouched.
But as far I can see, there is no such information available in the statistics.