You will probably be pretty safe putting YOUR (override) VERSION
of resources\
viewer.html' into the Resources folder into the CONFIGURATION FOLDER. Updates to that ORIGINAL file are probably low, so you will not need to update YOUR version.
For lurkers: Kovid designed much of Calibre to look into the Config folder/resources first and use that piece, if found. Calibre [U]Updates do not overwrite this area. /U] (the burden of keeping up if needed, is on you
)