I am not using a2x, but asciidoc and xsltproc. The attached manpage.xsl
imports the default docbook processing for man pages and then also adds
a "External References" section at the end containing all the links.
This has the style of footnotes, so it does not disturb reading.
Thank you! I was able to use your stylesheet successfully:
asciidoc -o man.xml -d manpage -b docbook man.txt
mkdir -p man/man1
xsltproc -o man/man1/inochi.1 manpage.xsl man.xml