I think you might be able to get it to work if you had a enough public IPs. i.e. One for the WAN interface and one for every LAN user. You might be able to do a 1:1 NAT for every Private LAN IP and add a Pass any WAN rule for every Private LAN IP. But I don't know for sure.
BTW, are you sure that pfSense doesn't support Vouchers?
http://69.64.6.24/index.php/Captive_Portal_VouchersRoy...