Interfacing tri-state logic across 3v3/5v. Do buffers mirror all three logic states?
No, typical buffers can't detect a high-Z input and put their own outputs in high-Z in response.

If there is a buffer that can do that, I am unaware of its existence.

If you want your buffer to have high-Z outputs, a separate EN (often active-low) input pin is usually used. 

If a buffer is enabled for output and has a high-Z input, there is a high risk the input will be detected as switching intermittently between high and low, causing the output to "chatter". This can cause undesired power draw, EMI, or cause whatever circuit is receiving the signal to go into an undesired state. Pull-ups or pull-downs should be used with high-Z outputs to avoid this kind of behavior.