How to properly grow frame with GPU effect?

I'm attempting to write a basic effect plugin for PP that grows the frame by x pixels in each direction. I've got software rendering working via the After Effects API. However I'm struggling with the GPU accelerated path (using the xGPUFilterEntry entry point). To clarify, it works perfectly if I don't grow the frame, and just render straight into the output PPixHand passed to PrGPUFilter::Render.

The documentation here says "If the effect grows or shrinks the output area (e.g. rendering a drop shadow), it is allowable for the effect to allocate and return a different sized outFrame."

So I allocate a new frame with PrSDKGPUDeviceSuite::CreateGPUPPix, with exactly the same attributes as the input frame, only with a larger width and height. This "works", except obviously PP doesn't know where I want the frame positioned, so it appears to grow from the top left corner instead of from the center of the layer. I was thinking I just need to tell PP the rect for the frame. But there doesn't seem to be any function to set the rect (unlike PlugppixFuncs::ppixNew?). I've tried manually setting the "bounds" rect on the PPixHandle but it seems to not affect anything, and the comment next to it says "always 0,0 origin".

So in short, I can grow the layer, but how can I tell PP to offset the grown frame by x pixels in the x and y directions, instead of anchoring the top left corner to the position of the original (ungrown) layer?