[SM] Nice idea, but the milling faces of millstones are designed for cutting and shearing the grains, not merely crushing them, so you'd have to piece together the shreds of clothing afterwards. More trouble than it's worth really. The drive home this evening was only 45 minutes - the best time for a week now. Some bits of Holland are frenetic. Luckily, I live in a nice rural bit :o)
What you need is not a grain mill, but an olive press. Of course it would be hard to get the oil stains out afterwards, but you could definitely get a good crease in those underpants. That would have the additional advantage of being drivable by donkey-power, so you could iron on a windless day.