From your problem description the usage scenario is not quite clear. Do you pass XML+XSL reference directly to PD4ML? In the case it should not work: PD4ML accepts as an input format HTML only (well, also SVG). Before a converting to PDF you need to transform the XML to HTML.
For a case it cannot load images/CSS by HTTPS:
PD4ML does not “officially” support HTTPS, however it implements very basic HTTPS resource loader (using the standard JDK classes). For example, WebSphere and Weblogic implement their own versions of HTTPS protocol, and the implementations are not derived from the standard API. That causes ClassCastExceptions on the platforms by attempts to load HTTPS resources.
In order to workaround such issues PD4ML supports so-called custom resource loaders.
If it is your case, you may implement your own HTTPS loader, utilizes Weblogic SSL classes. I’ve attached our standard SSL loader implementation, which could give you some implementation ideas.