PD4ML utilizes many AWT classes, so there is no easy way to implement GAE compatibility.
PD4ML architecture consists of two layers: HTML renderer and PDF output pseudo-device. The “pseudo-device” implements java.awt.Graphics which makes possible to “print” rendered document layout to a buffered image, for example. Or to capture any graphics output and to generate a PDF from it. A compatibility with GAE would require to sacrifice a lot of useful functions. Also I expect some troubles with TTF fonts.