how to hard-bypass the input of a line driver with a switch?
You don't need a resistor if you don't want to use one. Inside the DRV134 the input feeds an inverting op-amp configuration and that is stable with a signal (or 0V) being applied to that input.

The THAT1646 gives no information that I could find that explains what might be happening inside the chip. I would suggest that if you want more help you show a fuller circuit diagram and indicate precisely how you would interface to both chips - they don't seem to be that compatible having read their data sheets.

The data sheet says it is pin compatible but what realistically does this mean - maybe in very common configurations they can be interchanged.

Alternatively just try it - I don't think you'll damage anything. For anyone that doubts about the chip internals, here's a picture: -



Note that pin 4 connects via a 30k resistor to a virtual earth formed by A1 along with two more similar circuits making the input impedance 10k.