The Orca Search <http://www.greywyvern.com/orca#sear> can do this, but it
requires that you index the online copy first.  You don't need to have the
search engine located on the same server as the site you are indexing.

Then in the Control Panel, you just write an offline Javascript version of the
online search engine which only requires minimal modfication to work with an
HTTrack mirrored copy.

I built this search script and actually designed the offline Javascript writer
using HTTrack output :)