...and the .ico files for my retro fileshare now have higher-resolution versions for Windows 7... with the sizes carefully chosen so that XP will pick the dithered 98SE-originated 48px and below while Windows 7 will switch to the smooth ones at any size above that.
I wound up having to slice, dice, and redraw Windows 7's Libraries icon to get the kind of folder art I wanted to use for a base.