The market I go to near Philadelphia is situated under a stand of oaks. One of the dozen or so trees there had a banner year this fall so I picked some acorns to eat myself. It is a lot of work to make acorn meal - first the picking, then the cracking, then the boiling and mashing, and then a final drying, and then the cooking into a dish... as an occasional addition to my diet it is well worth my time though. Acorn meal imparts a rich, nutty, unique flavor to things like carnitas. It's also really tasty mixed with corn meal to make "Corn-acorn" bread. But alas, many of this particular batch were partly rotten. I guessed correctly the pigs wouldn't really care about some rot and worms so I fed them.
Now that I have photographic evidence my pigs have eaten some acorns can I claim to sell "acorn finished pork"? In a word, no. Finishing implies that a substantial majority or 100% of the diet be composed of the named feed, i.e. corn-finished pork, grass-finished beef, etc. I could call my pork "whey finished" since they do drink large volumes of it during the finishing phase.
Let's run some numbers - The tree I collected these acorns from had a heavy crop and I collected about 50% of the fallen nuts. I estimated yield to be about 120lbs from a tree with a crown that spread close to 50 ft. 50 x 50 = 2500 square ft. One acre is about 43,000 square ft. 43,000/2500 = 17.2. Round down to 15 trees per acre so they can get light deeper into their crowns... 15 trees per acre x 120 lbs per tree = 1800 lbs of acorns per acre, if one could find a bunch of equally productive trees. My neighbor got 4 tons per acre of field corn last year (others do even better). With a grove of oak trees could I get 1/4 the food calories of a tilled and harvested field? Both corn and acorns are mostly carbohydrate, so they're similar in nutritional value to a pig...
Of course, I've committed a pretty egregious error in the above calculation. That 4 tons my neighbor grew was dry. The acorns I picked had not dried down. And acorn shells are heavier than the indigestible parts of a kernel of corn. I think on a dry matter basis the acorn yield would be less than half the weight I collected - if we're in a mood to be generous we could call it 1000 lbs per acre.
But if we're going to continue in the vein of intellectual honesty we have to point out the fact that most oaks bear heavily every other or every third year, not every year the way corn does. So really over the long term the disparity of available calories is more like 20:1 (or worse). And that assumes we can find a really good oak grove. I'm into trees and know of a few productive oaks in Pennsylvania, but I can't think of anywhere I've ever seen a whole acre all bearing heavily simultaneously. This is not to say such a place doesn't exist, I'm sure it does. But skepticism is on order when somebody claims their their pigs "finish on acorns". If they have more than 2 pigs total I'd be pretty darn incredulous of the claim until I'd seen the trees for myself and the ground better be thick with acorns. With 80 pigs to feed, 100 pounds of acorns does not last long at all.