I gave that a shot and it doesn't seem to be working
For testing, I now have an inactivity timeout on the SSID of 5 minutes, I have the option to deauth inactive clients checked, and I have a reauth interval of 7 minutes. This is all with MAC auth on that SSID.
If I connect a client and wait >7 minutes, the client is still able to browse without getting the captive portal page again. It looks like if I totally disconnect and reconnect, I get the portal page, but I want this to take place mid-browsing if the reauth interval period is exceeded,