Scripto is an alternative to the ProofreadPage extension used
by Wikisource. It is based on Mediawiki but also on OpenLayers,
the software used to zoom and pan in OpenStreetMap.
The only website I have seen that uses Scripto is the U.K.
War Department papers, and in many ways it is more clumsy
than ProofreadPage. But there might be a few ideas that could
be worth picking up. Take a look.
The software is described at http://scripto.org/
As for reference installations, they mention
Lars Aronsson (lars(a)aronsson.se)
Aronsson Datateknik - http://aronsson.se