The following thread contain configuration for payment reminders on PPPoE: http://forum.mikrotik.com/t/mikrotik-advertise-url-with-pppoe-environment/35755/1
You can adjust that to work in non-PPPoE environments, too - just find some other way to populate the address list. If you’re not looking to just periodically show a payment reminder but to show it every time the client requests a webpage mark the connection as ‘payment_reminder’ in the first mangle rule, and don’t implement the second mangle rule as well as the only filter rule.