In general, the process would go like this:
1/ Navigate to the Finder.app application file.
2/ Change its icon via the usual [Get Info, Copy], [Get Info, Paste] method.
3/ Drag the old icon off the Dock, then drag the newly icon'd app into the Dock. You now have what you want.
Finder.app lives in /System/Library/CoreServices.
I haven't done this myself, but it should work. Does anyone know of any issues with this approach?