There is a similar -but with different functionality- idea in another thread:
http://forum.mikrotik.com/t/count-users-with-api/81240/1
Probably not what we want because the dchp lease script is not triggered every time a client is connected,
for example if the MAC address of the client exists at the leases table and the lease is not expired.
So the only way I see this working is by hotspot user login/logout script