That's intended. It's for overwriting defaults (although now that we install Gecko during wineprefix creation is doesn't make much sense anyways). You can't do that. The default is what's found by 'step (2)' from your description, not any hardcoded path. The version from Gecko package is meant for packaged Wine. You should copy the .cab file to appropriate directory for your own build.
Ok, so I'm wrong. Sorry for the noise in this case.