The first stage of your idea, having a cgi script talk to a daemon process and start up another process, is no problem. I've used IO:Socket for that purpose several times.

The real trick is getting updates back to your page asynchrononously as the child process generates them. Yes, you could have them happen only when the browser user hits a refresh button, but that's not terribly effective.

Without using something active in the page, such as a java applet or perl plus plugin I don't know a good solution.

Update: There are all sorts of ways to communicate to a running process on the same machine without a socket. You could use named pipes, signals, or write to a regular file it is monitoring. If it has a GUI, you can generate Windows events (on Windows) or X windows events (on Linux).