Appending to a PDF document at arbitrary position on its last page

My use case is about appending the rendering to an existing PDF document, with the special requirement of *starting such addition from the blank area left on its last page*, as shown in the following example (gray area stands for the trailing contents of the existing PDF document, green area for the blank space where the layout engine should start to build the rendering):

[`PageSupplier`](https://github.com/danfickle/openhtmltopdf/blob/open-dev-v1/openhtmltopdf-pdfbox/src/main/java/com/openhtmltopdf/pdfboxout/PageSupplier.java) seems the way to go, but it doesn't allow to constrain the layout to an arbitrary area (expressed as a rectangular shape or, at least, a vertical coordinate where to start inserting contents) within the supplied page.

Is it possible to achieve my goal with the current implementation?