At Apache Forrest we would be pleased to see this happen.
We have our own modified Cocoon 2.1 block for enabling this. Here are some clues that might help ...
Two snippets from our PDF Plugin's changes file ...
Updating the fop-library to fop 0.94 and
upgrading the stylesheets to work properly with the new and more
Replaced Cocoon FOP Block with special Forrest edition
"cocoon-fop-ng-impl-special" compiled by jeremias.
The sources are in the jar. See r603892 and the thread
Re: Plans for integrating FOP .94
Subsequent update to fop-0.95
The jar and sources are now at: