Without access to the pits/garage, your only real way of getting autographs is to look for autograph signings at either the souvenir trailers or stores around the area that might have signings. (Most souvenir trailer signings require a ticket that you have to get prior to the signing--usually the morning of the signing.)
Martinsville is a great place to get some truck drivers and a couple of the cup drivers. The best day is Friday. You can stand by the rail over by the motor coaches and get a couple of the cup drivers. The truck drivers will be coming down the hill around the same area. You may be able to get Dale Jr. by the helicopter pad.