Chris: If you disable the router's DHCP and are okay with using a spare computer as the DHCP server, there's software called "m0n0wall" that you could use. It's a BSD-based firewall/DHCP embedded OS package that runs off a CD and stores its data to a floppy. It'll let you set up a captive portal, with whatever page content you want, and let you code hard/soft-timeouts (i.e., (soft) re-display portal page after $x hours of a person's MAC address not being on your network, but do not display the page as long as they're active, or (hard) automatically re-display the portal every $x hours, regardless of inactivity.

http://www.m0n0.ch/wall/ (http://www.m0n0.ch/wall/) is the website, check it out!