FreeBSD like OpenBSD routes all power management related commands via the older
apm(4) interface.
There should be 2 device nodes in /dev related to apm, group write permission may be required to change such settings.
I'm not entirely sure, I don't really play with power management.. don't own any laptops.. my systems are always on, if I wasn't using them they would simply be off.
EDIT: All of the above is most likely wrong, just noticed that FreeBSD has several sysctl members related to acpi(4) and likely uses them instead, explaining why users cannot control this information. Sorry.