This project automatically adds static search to a pkgdown site. It's not a hack...it's 5 hacks. But it might possibly be useful.
cp -R docs/* _site/
TIPUESEARCH_ZIP=http://www.tipue.com/search/tipuesearch.zip
wget ${TIPUESEARCH_ZIP} -O search.tar.gz && \
tar -zxf search.tar.gz && \
rm -rf search.tar.gz && \
rm -rf __MACOSX/ && \
mv Tipue\ Search\ 6.1/ search_js && \
mv search_js/tipuesearch/ . && \
rm -rf search_js
bash add_search.sh