In fact HAL is used only by the Thunar file manager and some plugins for example to detect removable media and also by the core to shut down the system, which I couldn't get up working
For sure HAL can be dropped and replaced with udev as an event source for DBUS which is used heavily by Xfce4.
Xfce 4.8 development is ongoing, will check details. Anyhow, I think HAL can be dropped in the current 4.6.2
Just a side note, LXDE is not using HAL at all