Technically speaking, the "1 min/level" duration could be a variable effect, as it does change depending on your level, but I believe the intent of empower was only to effect die rolls, however the "affects half again as many targets" part may negate this interpretation.
But then again, I know this isn't what your looking for.
Greater Magic Weapon, +1 enhancement bonus per 4 caster levels. Variable effect, so Empowerable.
Magic Vestment, same thing.
Not much by the way of empowerable buffs, but I only glanced through the SRD.