The --include option does not work when using the docbook2man wrapper 
script.

Given the limitations of the man backend, this effectively rules out any 
kind of a workaround using --include.

/usr/share/sgml/docbook/utils-$VER/backends/man invokes nsgmls without any 
arguments.

I fixed this bug by exporting SGML_INCLUDE from /usr/bin/jw, then passing 
$SGML_INCLUDE as a parameter to nsgmls, in the man wrapper.

I'm surprised that this hasn't been noticed before.