I poked through the API, and there is no way for an item that has a stack size of 1 to spoil into some multiple of the current maximum belt stacking level. I would use this power to self mod promethium spoilage, so that a full belt promethium becomes a fully stacked belt of stone.

I'm sure some of the more complicated overhauls would also love to have such items, and to tie the productivity of the item's spoilage results to belt stacking research

I don't even know if the transport line abstractions would put up with a single item suddenly becoming a belt stack of items. and if it could, adding this would increase complexity bc the game engine would have to make sure it didn't create illegal belt stack of stuff that doesn't stack, this would probably require other complexity to implement as well.