it seems to me that the emery cloth/paper solution is at least one way for us to try improving our brakes, all be it that it will be a slow process.
A slow process indeed. If half the circumference of an 8"-dia. × 2" brake drum needed to have just 0.001" removed from it to make the drum round, that's 0.025 cu.in. of material. That may not sound like much, but it's the same amount of material as in a ⅛" length of ½"-diameter rod. Imagine how long it would take to remove that much material from a ½" steel rod by rubbing it back and forth on a piece of emery paper. That's how long it would take to make the drum round by manually removing "only" 0.001" from half of it with emery paper. Compare that with how long it would take to remove the same amount of metal in a lathe.
This is why motorcyclists who work on their own bikes, but who don't have a lathe, should do their best to become best of friends with someone who does.
I've been lucky (or prescient, or foolish, or ...) in obsessively accumulating tools, machinery and machining skills specifically for my motorcycle interests throughout my totally-unrelated career. As a result, this has reduced the number of times I have to rely on "Argentine engineering" to keep my bikes running. JB Weld, sheet metal screws, duct tape, etc. can work sufficiently well in some situations but, although emery paper might improve brakes in some cases, it only will accelerate the onset of arthritis and elbow-replacement surgery in others.