In my current project I need to have more fields on the inventory lines for my products: "InStockDemo" to represent stuff that's in stock, but on display in a physical store.

I guess there could also be different needs, like "In stock without original packaging" and stuff like that.

Have you considered making it possible to add meta fields to the inventory lines? Or preferably just make proper classes of them like we construct our content types, in addition to inheriting the IWarehouseInventory interface. Or something like that. As of now, all these non-standard scenarios (and I bet there are a lot of them out there) need to be custom tailored every time.