\Needs{"mkdir -p mytmp"}
\Needs{"mv \jobname.{4ct,4tc,aux,css,dvi,idv,html,lg,log,xref} mytmp"}
Sure, as long as \write18 is enabled, that's another way. If you're
only using tex4ht on your own documents, that's ok, but it's much too
dangerous to enable by default.
Best,
k