When I say that your arm would hurt, I mean that it might take 2 hours even if you just had a bunch of carts full of balls, and never once had to pick the balls up, and that within two hours that soreness would begin to really get to you. The fact of the matter is that you're right. Practice makes permanent, but at the same, practice makes perfect what you're doing. In other words, if I have an ugly, ineffectice, dinky serve, then after a thousand balls, it'd be really hard to get rid of. But my first serve percentage would be in the high 90s, because the nature of that serve is that it's killable, but hard to miss once you've used it for a bit. And after a thousand balls, you start to "embellish" your motion little by little, until it becomes still-ineffective, but the target, getting the ball into the box [I used a dink serve for almost a year before throwing it away completely, so I say the target with a dink is to get the first serve in from experience] would be almost completely met. With a more difficult serve, like a flat serve then your speed might go up after three days of 1000-ball stints, because it DOES build muscle to exert it, even overexert it, as long as you don't tear anything, and your accuracy might only go up very slightly after 1 day. But after two weeks, you would likely have embellished it to the point that you have the percentage of someone who frequently sliced it: not really any room for safety, per se, as a kick might offer, but not especially bad.
The idea that you have a high percentage after a week or so, perhaps you could move up to serving accurately; three traffic cones in the different zones, or whatnot.
The whole idea here, though, is theoretical. Perhaps in a few weeks I will test it, but not until I've gotten myself a week or so to recover afterwards, and a few days between training excercises, which I won't have until june. During that time, I'll write up a plan, and certainly make it public, though I don't think anyone would be very interested.
