I don't know what version of NW the Steam and slim browser use but if it is > 0.13 creating "PepperFlash" folder and copying the pepflashplayer.dll and manifest.json found in "C:\Users\<YourID>\AppData\Local\Google\Chrome\Use r Data\PepperFlash"there should be enough without any changes to the package.json or package.json.default.
If you copy the plugin you must do it again every time when the plugin is updated as it won't update automatically. If you change the flashDetect.js it will automatically detect the update and use the updated plugin.