Hi Jack,
Much appreciation for looking. I’m pretty sure there is nothing existing in the WordPress universe that can do this.
I’m baffled because it seems like such a simple thing conceptually. I do applications programming for CAD software in my day job and this would be a piece of cake.
But then I’ve looked at so many ecommerce platforms over the past two years that cannot do this–I did find, finally, Magento Go or Community has the ability and it’s simple and built in. Magento seems to be the only one and since this is a requirement for my store I’m going to be a Mag-girl.