How can I call/pass a parameter to a JSP page from within a scheduled LotusScript agent?

Here's the long answer, though:
There are some Web-related LotusScript methods, but they are all foreground (NotesUIWorkspace) and thus unusable in a background agent like yours.

You would be best advised to create a Java agent instead of a LotusScript one. I have myself created a couple of agents that connect to a URL and retrieve what's there, and my Java skills are extremely basic. Networking, connecting to resources over the Net, is built in with Java.

Staying with LotusScript, a method that I myself have not tried but which I am sure you could look at (assuming you are running the Domino server on a Windows platform), is to use OLE with MS Internet Explorer (which then needs to be installed on the server). You could get a handle from LotusScript to the Browser ActiveX component in IE and make it connect to your JSP. I am sure you could get it to work. I would be worried about the stability of your server, though. IE is a foreground application, and I don't like activating that kind of application on a production server. I'm thinking about memory leaks, security holes, crashes that prompt for actions and the like.

Finally, if you are on Domino R6 there is the option of using LS2J -- calling Java from inside LotusScript. There is a good introduction to that approach, a presentation from LotusSphere 2002.

All in all, I'd advise you to look at a Java approach to do what you need.