Does the Almighty Wiki say where this manuscript is found? I remember hearing a while back (on the 'Downs, I believe), that Hammond and Scull (or some other insane scholarly bunch) got their hands on an illustrator's guide in order to do their research. I'd just like to see the citation for that before I go around quoting it.
At any rate I know that
somewhere in LotR (because on one of my more recent readthroughs I was specifically looking for visual cues for the characters) Tolkien casually mentions Gollum's "garment". Doesn't say what it is, but that seems to belie the full set of clothing. This honestly could be Tolkien attempting to explain away various inconsistencies if while he was writing the tale he didn't have a clear, consistent visual image.
If you do want a "darker" Gollum, you could always go with the Rankin-Bass version...
