You could just spawn a bunch of radclient processes in the background, to simulate simultaneous connections. It wouldn't be terribly precise...then again, since radclient is single threaded, then it should be a one to one mapping (spawning 300 simultaneous background process would be equivalent to 300 simultaneous users). Of course, since your automating the connections, your going to hit the Radius server harder then 300 actual users, unless they are all authenticating at the exact same time, but it might meet your purposes.
If your into scripting, you could write a quick Perl script to also perform the authentication, but you would probably have to spawn child processes, or use threading, both of which are more complicated then the above, but may give you more control.