I think the problem here is more the fact that it is presently unclear (in etch I mean) that desktop-base indeed prevents KDM's customization the usual KDE way ("Control Center -> System Administration -> Login Manager"). Not only is it impossible to make the users list visible but it is also impossible to change the login appearance (other features such as passwordless users do work however).
Something like "Users who intend to customize KDM (appearance and/or functionalities) should not use the desktop-base package" should be clearly visible somewhere (I spent quite some time finding that I needed to purge the desktop-base for my settings to be taken into account). Regards.