I quickly hacked up a PAppletScriptlet, that lets you send it javascript which it will then compile and use as your Processing applet (IE: setup(), draw(), mousePressed()).

Quick Overview:
http://eventhorizongames.com/wiki/doku.php?id=articles:processing:scripting:start

Example site (NEEDS Java 6, and only tested on Firefox, I'll clean it up later):
http://jddaniels.googlecode.com/svn/trunk/processing-scripting/dist/index.html

It's a quick and dirty hack right now, I'm still trying to find a way to "share" the containing PApplet's fields and methods with the script, so that I don't have to prepend every method call with a p.*, or having to use the with(p){...} javascript construct.

I think the best way would be to possibly port the Processing parsing stuff to be done on the fly, because the JSR 223 scripting spec. and scripting engine project, provides for ways to compile java code on the fly, so PDE code could be emitted as java code and compiled on the fly (although I don't know how we'd then inject that PApplet into the already running one with all it's context etc....)