All I can say is we should not say it won't work IF we haven't tried it - as at close range, it did work in a .54 calibre pistol - which is much smaller than a 20 bore.
Now, the purpose of using buckshot in a 20 bore comes to the forefront.
Years ago when I was a kid and wanted to try buckshot in my 20 gauge shotgun for shooting foxes, the largest I could get was #4 buck. Not sure how many there were, but the number 28 comes to mind. Now, I do not know if that 28 was in a 20 bore shell, or a 12 bore #4 buck load?
One would have to weigh a #4 and see how many make up an ounce (437.5gr.) or perhaps up to 1 1/4 ounce, 546.8gr. and use that with an appropriate powder charge.