That's a Processing Implementations question (moved it). And what do you mean by "preloader" exactly?

For example, some processing algo's may take a while to process, whether for clientside to actually calculate/render or to load the required external graphics elements... Preloader would be something like your classic AJAX preload spinner or progress bar to give the user some sort of sign that their browser/page hasn't frozen, and is still "working on loading it". 

That's not a preloader, then, as these are supposed to work while loading the main program/library, which shouldn't take that much time.
It is more like a "busy indicator".
To my knowledge, JavaScript is monothreaded, so it is hard to make two things (animate the busy sign and do the computation) at the same time. A solution I saw quite frequently is to just display a spinning animated Gif: the browser animates it while the JS runs. But it must be in an HTML div (perhaps positioned absolutely) above the canvas.

The pseudo-preloader / animated gif... Is there a way to have processing send a signal to JS when it's finished loading to disable showing the pseudo-preloader?

Probably. I know how to do this with regular Processing (applets) but not with Processing.js, which I haven't really used.
It should be documented somewhere in P.js documentation; shouldn't be very hard, as everything is JavaScript running at the same level.

Just curious - how would you do this with Processing applets?