Continuing the soliloquy ... I don't know how Jean-Louis got it to work, but setting $ENV{MAGICK_CODER_MODULE_PATH} to the directory that contains the coders enables them to be loaded when the registry key either doesn't exist or specifies a wrong location.
(Earlier today I googled up a post that asserted that MAGICK_CODER_MODULE_PATH is ignored on Windows. Thankfully that claim turns out to be bullshit - at least in relation to the ImageMagick binaries that I've downloaded.)