I'm presuming regulations wouldn't be updated/waived quickly enough (for feed grade produce, etc), which is part of why famines of this level come to be in the first place.
It's not that stores wouldn't buy it or be willing to sell it, but that companies are much less likely to go against regulations than an individual, even in times of distress.
And to get to grocery store shelves, a product has to go through the hands of multiple companies, depending on level of refinement. But probably at least a producer, a wholesaler/packing house, a distributer, and at least three transport companies. All of which would need to be willing to take the risk of breaking regulations. Which is normally a good thing, since it keeps our food supply chain safe.
It's part of why EPA/USDA/FDA/etc regulations focus on what companies are allowed to grow or manufacture or sell, instead of what consumers are allowed to buy.
So what you're describing is true, but if we're talking about normally illegal-to-sell-food, depends on many people deciding to flout regulations.
This is all in addition to any rationing or other emergency rules that might be put in place (like attempts at government controlled distribution, ala Venezuela)
Failing to do so would take a huge number of people whose job description is to do exactly that. If one tries something new that works the rest is sure to follow.
They are puzzling all the time, how much eggs to buy? how much potatoes? how much this? how much that? They never get it exactly right, they are always a few bags of apples short or have to throw them out.