See:
bitcoin-qt.pro:    QTPLUGIN += qcncodecs qjpcodecs qtwcodecs qkrcodecs qtaccessiblewidgets
and:
src/qt/bitcoin.cpp:Q_IMPORT_PLUGIN(qtwcodecs)

Win32 links plugins in static.

Rather than changing behavior, I opted to take the (hopefully) less controversial route in order to get the buildsystem merged in more easily.