Perhaps a silly question, but why would you want to do this?
If you can threshold brake, the ABS would never/very rarely kick in anyway, and in mad emergency braking situations, it’s still there… Is it for drifting or circuit use that you don’t want ABS?
When I went to a driving school a while ago, they had a car setup with a homemade switch to turn ABS on and off in order to teach us how to threshold brake. All I remember was that the car had to be turned off then have the switch flipped (either enabling or disabling the ABS) and then the car was turned on again. I’m sure the car had a different speedo/sensor setup compared to Silvias, as it was a Dodge, but I do remember the speedometer working fine.