quick answer:
no
The reason for this is the following (more or less):
When a user decides to visit a web page he fires up his browser, send a request to the proxy. The proxy has a way to remember which computer asked for what and so on (typically a table storing things such as originating ip, destination ip, port number and so on..). The only way to figure out who's doing what is to examine this table, find the row that matches your interest. To do this, you need to have super user access to the proxy, and typically you don't - unless you're the admin of the network that is behind the proxy. If this is the case, then it's up to the software in the proxy to decide whether it's possible to list the current connections or not. If it is, you probably have to find a way to publish this list securely..
// Olle
