I'm not quite sure I understand what you would like to do. Are you thinking of injecting bits of code that could be carried with the "stream" waiting for the right trigger to execute? If so, what sort of trigger to you have in mind?

Delayed execution is a perfect application for functional programming. For a recent discussion and some examples, see Functional programming ?.

The specific application you describe for the web sounds a lot like a mashup tool. If that is what you had in mind, you might also want to check out Yahoo Pipes and the Google Mashup Editor (now a part of Google Apps). These are intended as graphical programming tools, but I believe the guts (especially for Yahoo Pipe) do something like what you are talking about. If you had something else in mind, perhaps you might add some notes explaining how your idea is different or goes beyond that?

Best, beth