Wrap your game inside Electron instead of NW.Js.
There is currently no support for Steam, but we have MadSpy in our Discord server (https://discord.gg/dZDU7Re) who is the developer of Steam4C2
We have already discussed about an integration into Electron and that should not require a lot of time. Though, he doesn't have a lot of time to make this possible. However I've successfully integrated Electron with the Steam overlay feature. I've never tried the Scirra's Steamworks integration.
This plugin/tool is also Open-source which mean everyone can contribute to it!
I hope I've answered your qustion.