A customer of ours requested this on our support ticket system too. By default we do not do custom post_auth development as this would lead us into a situation where everyone would ask us to code the strangest things for them (we have had some pretty weird requests). But we reviewed this particular request and implemented a means to allow 1 additional MAC address per account, although that second address has to be added manually by the server administrator. That is to avoid a security issue with automatic registration of MAC/UUID addresses on new accounts.
See
https://openvpn.net/vpn-server-resource ... -checking/ for updated instructions and updated post_auth script with support for 1 additional parameter. The page contains an example on how to add a secondary MAC/UUID address.
I'm still alive, just posting under the openvpn_inc alias now as part of a larger group.