The GIM6010-8 has an optional secondary encoder. I assumed it would give me a second position feed over CAN for redundancy, but I can't find it in the protocol. What is it for, and does adding it change the motor dimensions?
It isn't a redundancy channel. The secondary encoder exists to provide single-turn position memory after power loss. Because there is a reduction gearbox, one output revolution is several motor revolutions, so the secondary encoder helps determine the motor position when power comes back. SteadyWin's own wording: its only purpose is to remember the single-turn position after power-off.
Not readable over CAN. CAN gives you the main encoder position only; the secondary encoder position cannot be read. SteadyWin: "the second encoder is auxiliary only and its parameters cannot be read."
Dimensions: no change. It is an optional configuration and does not alter the basic motor dimensions or the output-side mounting pattern.