As well as being cumbersome, it could present a huge security problem if your users are typing their credentials on a Roku screen, which gets sent to some server. Unlike with a web browser where you have some degree of assurance that a web page is sending data (encrypted) using "https", a Roku user has no assurance that anything typed on a Roku screen is not being sent over the internet in plain text over an unencrypted connection for the whole world to see. I for one would not trust any Roku channel that required me to enter my login credentials on a Roku screen.
Roku recommends what it calls "rendezvous style registration" and use of an roCodeRegistrationScreen to have the user authenticate themselves on their computer then enter a registration code on their Roku. Some web sites (Google/Youtube, etc.) use OAuth2 authentication which, if it's already implemented on the server side, is very easy to implement in a Roku client. I'm not even sure whether Roku would approve public channels that send unencrypted plain text login credentials, due to the security risks imposed.
Here are some links:
Developer Guide - RegistrationDesign Guidelines - Linking ScreenComponent Reference - roCodeRegistrationScreenRoku SDK Examples (see register.zip)