aftermarket modifications

since the gas and brake are electric, this modification should be relativly easy. I have not heard of it in a prius. I have heard of it with other cars.

 

I imagine Toyota has a program to install these types of modifications. I think most carmakers do. Otherwise they should be able to point you in the right direction. But since there's no physical link for the controls, I imagine tapping into that wiring system would be easy enough.

 

I don't think it would be a good idea to do hand controls elecrically. The brake system in normal operation is electric, but does have hydraulics involved, for one to give you the hydraulic feel (which is done with the stroke simulator), and to have simple hydraulic control of the front wheels should the assist fail.
The accelerator has a dual hall effect system. Might be a bit tough to duplicate properly without triggering DTCs.

Installing controls on the Prius shouldn't be any more difficult than any other car.