We didn’t find one with a reliable behavior at the time we were building the original pi images. Many times on a mobile device, we found a captive portal would pull up a ‘log in’ screen, but that window didn’t have an address bar or browser functionality. Users would get trapped in it. It’s probably something that could be done and configured better nowadays, but we haven’t spent a lot of time keeping the pi deployments updated.
A volunteer, @jamesk has done a lot of phenomenal work on the pi platform lately.
Our RACHEL-Plus device does have a captive portal and a feature to turn that captive portal on or off.