Jonathan,
If neoclassicals and Marxists want to assign a portion of the value of an end product as arising from the capital goods that helped produce them, they can do so by dividing the depreciation for a period by the number of end products produced in that period to get the per unit contribution of the capital goods.
Regardless, capitalists depreciate the cost of their capital goods over time as an operating expense to smooth out their profits taxes. From an accounting perspective, there is no way around the fact that capitalists consume their capital goods, they do not accumulate them.
If economists want to theorize a different system in which capital goods are accumulated (which I don’t think they have to do, they can just admit that depreciation, amortization and depletion are ways to reflect the contribution of capital to an end product), that system is not capitalism. If they do choose to do the right thing and look to accounting tricks like depreciation to understand capital’s contribution to the cost of the end product, then the contribution of capital goods in all cases reduces down to monetary units, but I don’t know if that tells us anything about the value of capital’s contribution.
Personally, I think the reason why they insist on capital being a material thing is they want to avoid admitting that money and finance are central to capitalism. They want to continue in a world where physical commodities are bartered and no money exists, thus keeping the discussion in the cooperative world of industry and avoiding even acknowledging the predatory world of business (aka Finance). The irreduciability of material capital goods is what allows them to argue an outsized share of the value of the end product should go to capital.