Malte,
the xotcl request monitor can do this for you. You can issue from a connection thread eg. throttleThread do Users hits 494 to obtain the number of hits for the user 494 in the last 10minutes. If the user arrived newly, your get 0.
You can also get the same way the last peer address, or the list of the last requests of this user. It is quite fast, it takes around 100 microseconds per call on my notebook and half the time on our server with currently 500 users logged in.
The current version is http://media.wu-wien.ac.at/download/xotcl-request-monitor-0.27.apm
see also: https://openacs.org/forums/message-view?message%5fid=335571
