Hello Vincent, This is a very interesting question that your pose and something I don't think I will be able to answer completely, but there is some good information that I found. A press release by BrightScope on the 15th of October provides some details on the growing commodity investment selection within 401(k)s. The press release may be found at http://www.brightscope.com/about/press-releases/ and it will be third press release down. Other information I found was that roughly 9% of plans offered some type commodity investment back in 2009. The assumption would be that growth has continued and may be a few percentage points higher now.
For individuals that do not have access to such investments in their 401(k)s, seek out a trusted advisor to add commodity exposure through other means.