You can definitely implement an SSO without the "official" SSO extension. There is a few helpers most of which you pointed out that help with making an SSO easier to do. The only thing you really lose is the ability to immediately open the SSO in a new tab or in an Overpanel. Some of this you could do anyway, it just would require some extra work.
There is some written documentation that I would recommend you to read before tackling this project. Our Enhanced SSO documentation:
https://docs.q2developer.com/guides/frontend_development/enhanced_sso.html
This seems to be doing exactly what you want taking a normal SSR or CSR tecton extension and turning it into an SSO.
Hopefully this makes sense,
Trey Teter