I never heard of hardwood dowels causing splitting of a buttstock when used to plug screw holes in this manner. You shouldn't have anything to worry about.
I have seen stocks that split when lead shot is stuffed into a hole in the butt in order to add weight. Over time, the lead shot oxidizes and swells, eventually splitting the wood.
Would you please explain the mechanism by which lead shot 'swells' and exerts enough pressure to split the wood? For sure it oxidizes, carbonates and reacts with tannins etc in the wood but why wouldn't those soft surface compounds expand into the spaces between the shot? I've seen wood split over a lead plug in the stock but the cause seems to be drying and splitting of the thinner sections of wood on the horizontal axis of the plug, not the lead swelling to increase in diameter.
