How to use a fragment shader on a PGraphics in Processing 2.0xx?

So far I've only gotten the filter(shader) method to work as a post-processing filter on the whole window. While the syntax pgraphics.filter(shader) is allowed, it doesn't seem to work. The method filter(shader) works from/to the main PGraphics, so it would make sense that pgraphics.filter(shader) would work from/to a user-defined PGraphics. At least that's what I'm hoping, because I'd like to use a GLSL fragment shader on an offscreen PGraphics.

Is this possible? And if so, how?