I guess you could drop the Hotspot completely and authenticate via ARP as outlined here: http://forum.mikrotik.com/t/authenticating-clients-through-mac-address/49287/1
Then use NetFlow (TrafficFlow) with an external connector to collect information on all traffic flowing through interfaces, and configure it to not account traffic between subnets you don’t want to account for.
There’s no plug and play solution for that, though - you’ll have to set up your own NetFlow accounting installation. There are free and for pay packages out there for that purpose. The router only acts as an exporter of flows, the accounting happens on a completely separate external system.