I'm not an expert in casting "epoxymetal" but I have experimented with the aluminium and bronze fine powders (as sold for cold-cast fake metal ie sculptures). I have also messed about with using larger chips as a filler, by themselves and in combination with other fillers.
Pretty much any inert filler can be used in a composite. One thing you should watch out for is lubricant, it will make the bonds weak if there is any thing greasy on the chips. Also polyurethane is low adhesive so composite strength will always be weak compared to an epoxy which are generally high adhesive.
Another issue will be that the complex shape of most chips means that there will be a lot of epoxy by percentage, again that can make the composite weak.