Asciidoctor can generate DocBook, which can then be converted to mediawiki using pandoc. The results are quite good.
asciidoctor -b docbook document.adoc
pandoc -f docbook -t mediawiki -o document.mediawiki document.xml
You can even do it in a single command:
asciidoctor -b docbook -o - document.adoc | pandoc -f docbook -t mediawiki -o document.mediawiki -
-Dan