Why dpnt't we simply modify debhelper and similar tools to add this to postinst of packages: (only if upgrading from a pre-FHS version)
if [ $1 = configure -a { $2 is a previous version than 1.2 } ]; then
ln -sf ../share/$package /usr/doc/$package
fi
And it would be just adding a
dh_doc_hack 1.2 ( <- first version complying with FHS)
to debian rules...
