Captive Portal to allow specified protocol?

[g.moretti]

Hello,
i'm setting up a network having a headquarter and 8 branch offices, for the headquarter i'm using a supermicro 1U server with an atom dual core, 4gb ram and a 80gb sata disk, for the branch office i'm using alix 2d3 system, so very low cpu capabilities.
i already setup openvpn tunnel from al branches to the headquarter and i configured captive portal on all the branch office to restrict the web usage from the users.
the problem is that i need to enable always some specific protocol as the smtp or the remote assistance software (teamviewer, logmein…) is it possible to setup the captive portal to allow always that protocol or, block only specific protocol?
Thank for the help!