I did the trip on motorcycle 2009 in September trough Iran and entered Pakistan at Taftan border then to Quetta, continued over the mountains to Multan and then finally left Pakistan to India at the Lahore border crossing. Police escort from Zahedan to the border and then more or less police escort all the way from Taftan to Multan, which by the way was FOC.
I had no problems with security just a lot of hassle with all the check points along the way were one have to stop and show passport fill in your name, nationality, etc in a book. The real problem was the road condition especially between Quetta and Multan which was when I did the trip basically a motocross track. I don't know if the southern route from Quetta to Lahore is any better but it's definitely worth a try as the road can't be any worse then the one over the mountains.