Buffers output load in unpowered state
No, it is not at all safe to assume that the output pins are floating when the device is not powered.

I would assume quite the opposite: all pins are clamped to Vcc through ESD diodes. Since Vcc is 0V when not powered, that means the output pins are clamped to ground through these diodes, and you won't be able to apply a voltage greater than about 0.5V to them.

Having first made that blanket assumption, check the datasheet. Some devices may be designed with different kinds of ESD protection. However, the specific part you mention states pretty clearly in the datasheet, under "output clamping current" that the output pin may draw 20mA if the voltage is 0.5V below ground or above Vcc.