Doug pointed out this post to me yesterday. One of the comments caught my attention - it mentioned the Windows Direct User? Engine dll (duser.dll) which seems to be used by explorer to implement some windowless controls. Having recently played around with themed painting and discovering that uxtheme.dll (the documented source of themed painting) doesn't provide properly themed expando controls (the task lists in explorer) I decided to have a look into duser.dll.

Unfortunately while the exports list looks interesting, there is no documentation anywhere for this library. I plan on checking my Microsoft contacts to see if there's any way of getting at least a header file, but if anybody is listening and can help out then contact me ASAP.