I tried to find this out a while back. I stopped looking after I ran across a thread over at XDA that said it does not actually rely on a dll or the bmp files in the Windows folder.
I will look for that thread when I am on a PC later, but I seem to remember them referencing a registry location that they discovered which contained a binary representation of the actual icon.
I agree that it seems like it should be a simple and possible thing to do, but I know I wasted several hours over several days trying to do this on my TP1.
I'll post the threads later if I can find them.
**EDIT here is one of the links I was speaking of -
http://forum.xda-developers.com/showthread.php?t=539909