You can try to install some additional software which works on top of ubuntu do some magic with ports ( dynamic http reverse proxy) might need a *load of resources ( just be sure nothing is touching zentyal)
Im currently trying to find captive software that suits my needs. But the most tools i found have to be on dedicated machines.
Or create a dedicated server in the same LAN and point zentyals DHCP gateway towards the dedicated machine which will do the rest. Ldap can be used from zentyal if what a login feature on your captive.