the servers and clients are not on the same LAN segment. capturing MAC has nothing to do with this scenario.
You haven't exactly told a lot about the network you want to setup. The logic thing is to authenticate against the firewall connected to the same subnet - and that will know the mac address. The same setup is assumed in the scenario using pfauth (or is it authpf).
Also, unless you are going to give a lot of instructions to people on how to configure their network, you will have a dhcp server on the same subnet - why not let that also do the web service for user management?
You haven't told either, how people connect - is it wireless or wired? Some access points supports that people authenticate WPA+something and the access point will verify against a radius server. And there are other possibilities depending on your setup.
But whichever way you setup your network, I think the best solution is if people establish an IPSec tunnel to the firewall, such that all traffic not destined for the local subnet must be tunneled through that. This gives you maximum control - you can even setup your firewall so that traffic coming in on a IPSec tunnel is also filtered.
Cheers, Erik -- Ph: +34.666334818 web: http://www.locolomo.org
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