I actually thought about all of those, but I had citric acid in mind mainly. It would probably have less impurities/extra ingredients than vinegar. Or maybe sodium bisulfate/pool supply pH-minus. But I can't experiment until I get some dateless buffs!
It's worth a try. I do have some of that on hand, and might give it a shot at some point. But ordinary white vinegar is plenty pure enough. (Don't use cider vinegar; it does have staining impurities, and it's too good to spend on metal etching!)
I don't know why all these experiments are necessary when we know that Peroxide and Vinegar do a great job and leave a nice looking coin. I use 40 Volume Peroxide (clear) and Vinegar. Here are some examples. I had tried another localized product on the 1891 Liberty. That's why the date area is discolored. The dates on the 1911 and the 1891 just jumped out in a matter of minutes.