how can I print full page watermark backgrounds

You have three choices of which the first is within the Pages document itself, and the others are post-processing of either PDF or printer output.

1. Within the Pages document as a variable opacity image relegated to the document background via Section Layout.
2. An Automator workflow that applies a watermark to a PDF of your document
3. Pages print panel option to watermark (and in my opinion) the least desireable or customizable.

Example 1:

Example 2 (using Automator PDF Watermark PDF Documents):

Example 3: (pretty much ignores any of the Aa selection choices to control font type, size, opacity, etc.)