"As an example - if you change brightness at the login manager, what should the osd look like?"
I would have thought that would be obvious - it should appear as the amount of light being emitted by the device's builtin display device increasing or decreasing independently of the image being displayed (assuming a backlight on the aforementioned device is currently present, active and is capable of being controlled by software).
Just as it does in KDE, Gnome, wmii, ratpoison, xfce, a virtual terminal, etc. After all, that policy is defined by scripts under /etc/acpi (on Debian) and does not require duplication in the login manager.
I use xdm as my login manager and I would call that lightweight. Sure, it doesn't solve all the stuff that gdm or kdm solve, but that's because it doesn't need or try to. Works for me, but it may not be suitable for everyone.
Re: Reusing desktop policies
I would have thought that would be obvious - it should appear as the amount of light being emitted by the device's builtin display device increasing or decreasing independently of the image being displayed (assuming a backlight on the aforementioned device is currently present, active and is capable of being controlled by software).
Just as it does in KDE, Gnome, wmii, ratpoison, xfce, a virtual terminal, etc. After all, that policy is defined by scripts under /etc/acpi (on Debian) and does not require duplication in the login manager.
I use xdm as my login manager and I would call that lightweight. Sure, it doesn't solve all the stuff that gdm or kdm solve, but that's because it doesn't need or try to. Works for me, but it may not be suitable for everyone.