If developers can run httpd and workstations can't, then setup those workstations so that they can't install any applications. Problem solved.
To perform what you want, Nagios would be starting and stopping every 10 minutes, as people power off/on there workstations (if you are using dhcp).
Really, though, think about it. You want Nagios to search a subnet for new ip's, add them to it's config files, restart nagios and report what? Report that there is httpd running on a workstation? If that is the case, simply make it impossible for them to do so.
Now, if you are triing to get nagios to configure itself, by searching for all the servers/workstations, I suppose that is a noble effort. But I fail to see how this is productive. Wouldn't it be a good idea to find out how your network is setup and what port's on a switch your server's are plugged into? Then configure nagios to monitor the ports and also monitor the servers. I assume that these servers are for production, so they should always be powered on, always be online.
I kinda get what you are attempting, but it seems kinda like wishful thinking.