That part of the explorer window has very specific limitations which is why we have never exposed transparency for it. Going forward with Windows 8 I suspect you will see a lot less use of transparency in the background of windows in general due to a significant change in the OS.
Thats not to say it isn't something we will be considering for future versions of WindowBlinds, but if it shows up it will be part of a more major update.
I guess it would have to be in an update of SkinStudio
too so we can skin it accordingly to the rest of the Blind being worked on.
And as I said "Windows 7 will hang around for a L O N G time yet and I therefore think this is a fair wish for future skinning this OS."
Thanks for noticing the thread Neil.