I think it's a matter of simply adding the required attributes to the default user class. user/register creates an empty user object and as you say the attributes are looped and displayed.
I'd recommend you do some reading before going too far. Check out
http://ez.no/doc/ez_publish/technical_manual/4_0 & http://ez.no/doc/ez_publish/technical_manual/4_0/concepts_and_basics/content_management/the_content_class for an understanding of content classes.
To edit the user content class you need to
Click on the "Setup" Tab
Click on "Classes" from the left menu
Click on the "Users" Class group
Click on the Edit Icon next to the "User" content class You can now add the checkbox attribute from the Attribute selector at the bottom of this page.