I need something different than the usual hotspot and I'm having difficulty figuring how to do it.

This is going into a seasonal trailer park. Currently clients arrive in the spring, power up their CPE, call us up to tell us they have no internet, we then ask what the last day of their intended stay is, set an appropriate comment on the gateway so that a script on the gateway router shuts down their queue on that day. However, we want to automate the process so that the clients don't need to keep calling us.

The network is not entirely bridged so MACs are not necessarily preserved as the traffic passes from an access point through the network, but IP addresses on all CPEs are static and are preserved. A 1:1 NAT converts the static IP addresses on our network to their public IP addresses. Some clients are year round but a login once a year to confirm email address wouldn't be a bad thing.

What we want is for the clients to power up their CPE, get a web page where they select the day and month they intend to be gone by (plus confirm their email address), and then enjoy the service. No username nor password required. The gateway will need to store all of this information on-board either in files or in comments. Our home-built NMS and automated billing system would then query the gateway when it detects the client and obtain the necessary information. Because the server is on a different UPS from the gateway and power failures are all too common, our script cannot rely on the server always being there but our server can always rely on our gateway being there.

I'm not seeing anything in the hotspot documentation that looks remotely like what I need but I am seeing bits and pieces that indicate that something like this might be possible. I'm pretty sure it needs the hotspot/walled-garden functionality to block access until the client "signs in" but how to link that to IP addresses and how to do away with usernames and passwords is very much not obvious. So any help would be appreciated.