Typewriter keyboard matrix control with Arduino
It is likely that the matrix is driven with 5 volt signals. If so, you could connect the scanning signal lines (rows or cols depending on the typewriter) tp input pins on the Arduino. You'd also connect the feedback lines (row or cols- the opposite of which was used for the scanning) to output lines on the Arduino. 

Finally, you'd write a sketch that watched the incoming scan lines and would light up the corresponding feedback line whenever it wanted to type a character. 

Make sense?