As promised, a quick update:
Ran the unofficial mod to add a nodogsplash captive portal to a vanilla librarybox installation on the tp-link3020. With a few edits to the nodogsplash splash page and to the librarybox contents folder, I managed to get a pretty robust setup going, that worked well across several OS. iOS is super smooth (at least on first try), but a bit slow, android is hit or miss, but android 6 seemed to work pretty reliably. android simply has more options for user error to enter the equation, as there are multiple "accept" or "dismiss" screens popping up, plus users can configure their WiFi to follow user defined rules.
Here's what I did:
1. edit nodogsplash splash page to include a brief explanation "you are not using the internet" and a call to action "I understand" that triggers the activation link.
2. create a new index.html in librarybox content that displays yet another call to action and points to start.html
3. create a new start.html in librarybox content where (finally) the real content is displayed.
Now, this is already a pretty robust setup. First fallback was to ask users to enter the hostname as an address into their browser. The last-resort fallback was to tell users to enter the IP-address for the librarybox into their browser.
Any other ideas for improvement, feedback?