Just a note that the code I referred to above is available here, along with a detailed readme.
Please note the warnings in the readme. I would not recommend you set this up unless you understand what this implementation is doing.
I plan to make a multipass integration (the official Shopify SSO solution) in the near future. If you’re interested in the multipass integration and want more concrete details you can PM me or reply here. If there’s signifcant interest I’ll prioritise it.