sleep-display for battery vs AC in GUI

OS: Ubuntu 12.04
Hardware: ThinkPad X230

Rationale: Being able to set different screen-inactive-times (->blank and lock) when on battery vs. AC is quite useful, for example 3-5 min vs. 20 min.
Earlier versions of the power manager settings allowed this distinction, the current one (System Settings -> Brightness and Lock) doesn't anymore (only for lid/critical actions in System Settings -> Power).

Observed behaviour:
The corresponding dconf keys are
org.gnome.settings-daemon.plugins.power.sleep-display-ac
org.gnome.settings-daemon.plugins.power.sleep-display-battery

Editing these seems to work after rudimentary testing, but also changes the "Turn screen off when inactive for" in Brightness and Lock to "Never", regardless of which values are set. Changing the latter in turn sets both keys to the same value, which destroys the customisation.

Expected behaviour:
The Brightness and Lock should partially have a column for AC and battery each, like the Power settings do.