Hi there,
Thank you for reaching out to us!
We are trying to reply and provide some information to older posts, in case others encounter the same situation and find it useful.
Based on what I was able to find, we currently do not support sending the login_hint as a query parameter in a SAML Connection, at the moment it is being encoded and passed through the SAMLRequest parameter.
I do encourage to post on our https://auth0.com/feedback page on this matter, as it is being monitored by our Product team and others can vote, if they would also want to see this as a potential feature.
Have a great one!
Gerald