I totally get what you are saying, and I'm curious as to how this will pan out. I think it may be that scaling on the resistor accuracy is the inherent gotcha... certainly seems to be the kind of thing I end up learning after coming up with some grand scheme in my mind.. LOL
I've always liked the idea of measuring the charge time of a capacitor against a comparator. Not sure if this is a used method, and I suspect sample rate is the gotcha there, but I'd always thought that a comparator vs. a known quantity ought to be darn accurate. Comparators are built in to the Arduino also, but I don't think the language addresses them directly, I think you need to go under the hood for a few lines of C and fiddling to use them.. though probably worth it, if only for the fiddle factor. Kinda surprised comparator functions aren't part of the language actually, now that I think about it..
1000% on the "Of course someone else has done it better, cheaper, faster, and thirty years ago to boot!" -- I have a great passion for re-inventing the wheel, perhaps you'd be a fan of my most recent design, it's rectangular so it's easy to carry....
Arduino (in my humble opinion) is all about "doing it the wrong way" (well, at least be safe, fire is bad, ummkay?) and learning from it. Take my advice and order some spare 328's, you are going to cook a few of them. Cheap lessons at $5 each. In fact, a couple of times I've seen a decent price difference downward on 168's, and I'd think the same happens with 88's. The 32-16-8 refers to flash program memory, otherwise the chips are largely interchangable-- so if you sketches are small, and you are ending up blowing chips, might make sense to get a few of the cheaper versions if the price point is right...
A thought-- maybe you could revise the title of the post to be more clear.. swarms of broke hobbyists seeing something like "free money" or "Tech Dumpster Around Back" kinda chums the waters, lol.