US Treasury 30 year bonds are currently priced over $102.... Calling a bond an IOU doesn't really make sense because you neglect the coupon payments (annual interest). So even though you're paying over $100 to get paid back a $100 principal the yield is still positive because of those coupon payments. Negative yield bonds also exist and are bought for various reasons.

"There isn't an institution in the world that could sell an IOU $100 for $100" is not true

That's a good point, but I think you can get one-year bonds with an annual coupon: that means you get one payment a year from now. So bonds in general aren't IOUs, but there is at least one bond that works like one.

You are right that would be an example of a bond/bill with a 0% coupon rate. There are still weird negative interest rate conditions where it makes sense to buy an IOU for more than you get back (not that I understand those conditions :) )

I think the 'weird' conditions are some combination of there not being other 'riskless' (or less risky) investments and certain institutions being legally required to hold a certain number or amount of 'riskless' or highly-rated (according to the relevant regulators) instruments.