All sneak attack really is is hitting someone in a vital area, one thats assumed to be normally defended well enough not to allow extra damage.
Its not out of the question to think of a Barb/Rog who rages and instinctively hits those vitals as the opportunities present themselves (ie flatfooted, etc). Every rogue has his own style. While a straight rogue maybe swiftly and carefully stabbing with his rapier in the spleen, the barb/rog is probably just slashing open throats and chopping heads with a greataxe.
Theres no limit on what weapons can be used for a sneak attack (as far as melee), so I don't see how one can limit them. You explain how a greatclub or a spiked chain can be used to strike descreet vital areas in an instant. Sneak attacks are really just hitting something important when someones distracted or not fighting at 100%, not the backstabbing of old.