I haven't done anything as elaborate as what you are doing. I have however built a portal that required a username and password that then dumped to a text document. I did this for a demo at work to show why people should turn off their wifi while at work ... For my purposes I used the EvilPortal module.
Since the NANO would be routing to the internet via the next hop, you should be able to load dynamic items in the landing page.
You also need to use DNS spoof to force the traffic to the portal.
You MAY have better luck with an AP thats made for the captive portal setup. I know CISCO made a captive portal device that worked pretty well.
Also, another suggestion, if you were to try to deploy this on a large scale, then you may want to investigate the NANO. It's got a bit more horsepower, and would possibly get more clients connected as it support 5GHz as well.
NANO to re-transmit wifi through captive portal
in WiFi Pineapple NANO
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