I agree that the system message that you need to read and accept when you login is the best idea for this.
You should be able to have as many as you want, with the board rules always being displayed first, then the next page displays either the first message, or they are cascaded so as not to annoy people that don't logon that often.
OR, perhaps those system messages could have an expiry date tagged to them, so if like someone suggested it was for an offer, that if that offer was timed limited then you could set that message to expire and thefeore not display.
