I somethime do straight arm pullovers as a way of tightening up the lats for the bench press.
Biomechanically, they shouldn't actively hit the serratus as the serratus actually tilt the scapula up not down. Serratus should in theory be hit by incline bench shrugs-arms extended in an incline press, shrug you shoulder up off of the bench. Light weight.
Also, I can only do pullovers with a neutral or curl grip. With a straight bar I get some feeling of dislocation. Same with a dumbell. I prefer to grab the sides of a 25, 35 or 45 pound plate. You can use a pair of plates flat to flat if you can handle them.