Default repo management in Maintenance tool settings

With an online installer, the repo used during installation is locked in the Maintenance tool (impossible to remove or even just change the login/pw). Is there a way to avoid that?
Say I want to use a temporary user name for installation for instance, then change it when the software is installed.

And I have two repos: one for stable releases and one for beta. I want some users to be able to switch between the two.
I specified both in the config.xml with the stable enabled by default and the beta disabled. I can install the software from the stable repo, giving login/pw when required. But then in the Maintenance tool, the stable repo has the login registered (and locked) and it works, but the beta doesn't have credentials registered and I cannot add them. It would only work if it was not access protected with Username/Password.

For both of my problems, if I could just remove a "Default repository" and recreate it with a different username, I guess it would solve it.