Switchback has ABS.  I would like to link the brakes.  Front brake also applies the rear.  A bit.  Simple idea, brake line with oneway valve and some restriction from front before ABS valve to rear, before ABS valve.   ABS still works.  Probably illegal...

 Didn't Moto Guzzi have this set up years ago?  The lever used to activate one of the front disks and the foot lever activated the rear brake and the other front disk.

FatRob.

Moto Guzzi had linked brakes as described years ago and in fact still have linked brakes. Having owned a couple of California's in the past I loved the linked brake set-up. I would think that what you are proposing to do may be a little more difficult than you envisage. Maybe find a wrecked, late model Guzzi with ABS and graft the system onto your HD. Good luck...

I think the Honda Blackbird had it. Proportioning valve adjusts to suit load on bike? Makes it too hard to do a burnout.

My prevous bike (still got it) is a BMW K1200R. It has linked brakes and ABS. I like the system. The Harley, being a heavier bike would benefit from a little bit of back braking. Looking at it as a safety feature too. Don't know how to improve ABS braking without adding braking force from another wheel.   Does anyone know if the ABS needs to have the brake lever pushed to be active.?Don't have a million dollars...