always depends on how your drawing looks like what method is the best.
not every solution works. pdf export can get problematic with to many objects on the screen. the other solution would be pixelbased output. There is toxis tilesaver that could be used, or you are drawing into a large PGraphics, both might work in your case.

The problem with pixel-based output, as far as I can tell, is that it's limited to the resolution that I'm viewing the applet at. I think that PDF/SVG output would be best as they are resolution-independent. I'm not too worried about creating a PDF file with many objects - I know that it will be slow to open & edit but that doesn't matter to me.

it is not limited., thats what i said. there is the tilesaver that saves out different tiles and stich them back together in the end. i exported images with a resolution up to 15.000 x 10.000 pixels without a big problem...
Same but different with the PGraphics. You draw into a buffer that could be much larger than your screen. if you still want to view what you are drawing, you just draw the resized buffer back to screen...

i also tried  a lot of pdf exports that worked, but at a certain file size, you just wont be able to open them with photoshop or illustrator, even when you are patient.... But like i said, it really matters how you draw your image and what is drawn in it... its hard to tell. if you have a small preview, i can recommend one method if you want to.

OK, here is an example sketch that illustrates the problem: This is a simple sketch that generates a new image with three blurred ellipses every time you press a key, and if you press 'p' it writes out a PDF file. The problem is that the transparency of the PGraphics object does not carry over to the PDF. The pdf output looks like this:

how many of these blurred circles do you have ? how many pdfs overall do you plan to export ?

ok :) if it would only have been some, i would have had another idea, but in this case, i wouldn't recommend pdf export. you might already get into trouble opening the file with several tens of thousand of circles, blurred or not.

so maybe check out the tilesaver, might work for you...
http://processing.org/discourse/yabb2/YaBB.pl?num=1248245155/11#11

you have to use OPENGL, thats true, but if your 2d sketch works in opengl, this would be the best solution i guess.
i exported an image for the following project with  a resolution of about 10000x15000 and it worked fine. The image had way to many lines for pdf export. http://onformative.com/work/projectm-cover-01/
only thing you have to keep in mind, if you draw lines, you have to change the strokeWeight, because it wont change when zooming in with the tilesaver.

You mentioned that it might also be possible to draw to a large PGraphics object offscreen .... do you have a link to any example code that shows this approach? I have lots of memory to throw at the problem :-) and it sounds like that could also be a straightforward approach.

no example to show. just used that approach for another drawing.
its useful if you draw something over time without redrawing the background, in this case the tilesaver approach wont work.
in this case you create a PGraphics with the desired size and just draw everything you need in there and use pg.save() to save the PGraphics to disk... Problem here, you need to recalculate and resize all your variables related to the zoom factor.

np. and good luck. let us know if it worked and maybe post your results in the exhibition forum, would be cool!