I looked into that briefly yesterday. Maybe I should have posted my unsuccessful findings?

The main reasons that a digital pot will not work:

1) Smallest resistance I could find was 1k, with a wiper resistance of 200 ohms!

2) Pot terminals cannot go outside of power supply rails, i.e. 0 V - 5 V. The RS485 voltage levels are well outside of the positive and negative limits.

3) Large resistance variations.

- Lee