While working on a micro-controller replacement for the MM5740/AAE keyboard decoder used on the Datanetics keyboard, I discovered a way to send the unique Apple 1 backspace keycode (0x5F). You need to connect line X4 of the matrix to one lead of a blank keyswitch and Y3 to the other lead of the same keyswitch. Then by holding down shift and pressing that blank key, you can generate the 0x5F needed to backspace the Apple 1 monitor.

Note that you could also rewire the left arrow key of a MM5740 based Apple II keyboard to accomplish the same thing. Unfortunately there is no easy way to avoid the requirement of pressing the shift key to generate the correct code.