Euhm, sorry to say, but I believe using the inactivity timer will not be enough here.
1) WIFI4EU wants you to recognize previous users with but enforce the portal again after midnight or after at most 12 hours.
This means that if I connect at 8am, I should have to reauth when connected after 8pm. With your inactivity timeout I can remain connected endlessly as long as I stay connected without ever seeing the portal again.
2) inactivity timeout is also not sufficient to not trigger the portal again. If my client behaves correct it will send a disconnect message to the network negating that timer.
If the user then reconnects (even if only a few minutes later) he will be presented with your captive portal again.
IMHO that requirement pretty much dictates a Clearpass or similar solution to properly adhere to that requirement.