The car has a cable 
By-wire systems would be much dificult to do this, some have a cable as a backup system though.
Anyway, the level of cable pull by the servo would be very small (I'd say about 20% of the total acelerator travel). You don't need much power to make the engine rev when desengaged.
Is danger envolved? Of course, it's a car, and a diy system is controling the friggin' accelerator!
(If skynet becomes a reality I'm doomed for sure!)
Edit: I've just remembered, I can test the whole thing on my Logitech G25. It has everything I need, gear selector and clutch sensors, I just have to take a speed reading from the simulator (this can be done in LFS racing simulator), and install the servo on the accelerator.
G25:
http://www.gamespot.com/users/MorkkiTH/show_blog_entry.php?topic_id=m-100-25314490
LFS simulator: