In XSL-FO, there are properties you can put on a fo:block-container to position it relative to the page boundaries rather than relative to its preceding block. Something like:
<fo:block-container absolute-position="absolute" top="3in">
<fo:block>
...
This will place the top of the block-container 3in from the top of the page.
I think getting that work in the titlepage system might be hard. The titlepage spec file can only specify one element, but here you need both block-container and block. So the spec probably has to specify the name of a template, and the template must be written to output both elements.
Bob Stayton
Sagehill Enterprises
bobs@sagehill.net