I don't think I've seen that as a requirement before (I must have missed that other post you linked to). I know we never developed a modal version of that form because we didn't think there would be a use-case for it (but then we were only thinking about a flow where the entire browser left the community and came back).
With the SSO registration form, you are coming back into the community from the SSO provider site, so if the entire browser has been redirected, it wouldn't make sense for a modal to pop up when the entire browser is redirected back to the site.
If you want the SSO redirect to happen inside of an iframe, you could write a custom component that starts the SSO flow inside of an iframe in a modal pop-up (you could probably use the @Modal directive to pop up the component that contained the iframe that kicks off your SSO flow) and then it would come back to the SSO registration form (still in the iframe) when your SSO flow redirects back to the site. I don't think Lithium has an out-of-the-box component that you could use for this, so you'd need to write something custom. You could probably write it all using Lithium Studio, but you might want to engage with professional services to get the benefit of their SSO expertise here (there may be facets to this that I don't understand or have not considered yet).
I hope that helps, and would like to learn more about your use-case if you care to share it.
-Doug
