On premise SSO web application

I am not totally sure about this.  You have the on-prem web app, which I would have thought meant using the Application Proxy, which I linked to previously.  Then https://docs.microsoft.com/en-us/azure/active-directory/application-proxy-publish-azure-portal into Azure portal and you could assign the guest users to the app accordingly and provide B2B access in principle.  

There could be a lot more to it than that, for example, you mention SSO but I am not sure how that works in this case.  Personally, I'd test this with a Proof of Concept or ask Microsoft this question directly, if you have means, via Azure support etc.  Also, by the way, there is a dedicated space for Azure AD B2B Collaboration, that's worth checking out. Good luck.