use triggers that are controlled by radio commands, with an ambient custom noise attatched (ie the typing). When you get in the computer 'trigger zone' enter the radio command (0-0-1 or so) and then the typing sound comes in,lots of on screen text comes up saying someything relevant to whats going on, (on a timer), when the timer counts down, script ends, computer is 'hacked', typing noise stopped, and user control is handed back to the player. Job done.
Next step... get someone who can script that, cos I sure as hell cant. Theory? yes, that I can do. Actually writing the code? yea right....
Good luck... sounds like a good mission objective. (although just a glorified steal the UAZ mission )