I've often wondered about the absense of belt buckles, watches, buttons, gold fillings, rings, and such with the bones. I think it likely you're on to something. There maybe a theory that these missing items were transferred to New Britain for safe storage but I think your explanation is more probable. 
It may be that the natives might have thought this common and celebrated tradition was wrong, and therefor might have been less then forthcoming about who, when, and what they saw. Or, they forgot.
Leon
I've been mulling over what may have happened to items such as this and these are my thoughts based on what i've read on the site.
1. One thing is we don't know how meticulouse the original search was.
2. As the casterway is eaten some of these items may have been dragged off by larger crabs and are distributed around the site and not specific to where the remaining bones are.
3. Clothing rapidly decays in a tropical environment so by the time the casterway got to the 7 site they may have had less clothing.
4. Belts would be used for other things rather than keeping your pants up so buckles may be elsewhere.
5. Reading Doctor Kings Book "shoes" i'm worried by it saying that in early expeditions the chopped 'vola' was dragged from site. In my minds eye I can see an object caught up on a branch and dragged away. Hope i'm wrong though.
6. The 7 site is bigger than first imagined and not all of it has been searched, just look at how new features were found just last expedition.
Note. No science has been used just my muddled old thinking. Do not assume anything being said is right, wrong or just pure guess work.