On the custom built, front I am not convinced that there is any time saving to be had. If you look at the speed of a ragtime/concert pianists hands, they go like lightening. If you insist on custom built, two digital pianos mounted close together, offset horizontally would do for starters. You might want to control the sustain pedals for each keyboard independently, or add some Midi controlled Organ foot pedals to do the left hand chords.
These midi organ foot pedals could be used with an acoustic Yamaha, one of those with the player piano facility, so that midi out from foot pedals plays the base chords to accompany the right. Watching my uncle on his old Hammond, the left had seemed to be changing switches most of the time.
Going back to an unmodified acoustic piano, the damping on mine stops well short of the high treble end, which gives some clue to a zoning technique using the sustain pedal to hold chords, and then while playing the melody with the right hand, pick out extra notes from the sustained base chord to beef it up and get it resonating a bit more. Someone with more experience might be able to fill in detail here.
Which gives me an idea, to "hack" the sustain on an acoustic into two zones, middle and lower, add an extra pedal, and to fit latches onto the pedals, so that if you hit it hard to the the floor it would latch and hold. Hmm.
Topic: Right hand only...