Is it possible to get my apache to display something like a
"503 - Server it too busy" page
when the number of visitors exceeds a certain number?

I've seen this message when visiting other sites sometimes.

There are a number of config directives in the httpd.conf file that sound like they'd help.

MaxKeepAliveRequests
MaxClients
ListenBackLog
...

Ive tried changing everything to 1 and testing it by refreshing a few browser windows, but the connections just seem to hang around and get served eventually rather than display a busy message.

If anyone knows if how to make a busy page display (or if its even possible on apache) your feedback would be appreciated.

My guess is that you really need to flood the system, send a ton of requests at once, maybe write a script that does a while loop with the GET command, in bash it's as simple as:

while true
do
GET 'http://localhost'
done

see if that works for ya, once its run for about 20 seconds try to access the site with your browser.

You can edit the amount of connections in the httpd.conf . That will fix your problem.