The lactulose/mannitol test only measures the permeability of the small intestine so cannot rule out increased permeability of the colon.
Although the colon (large intestine) can also be leaky, I think leaky gut studies and research generally examine leakiness in the small intestine.
I have been PubMed searching for studies that specifically looked at leakiness in the colon, and there are relatively very few studies that focus on the colon. Here are some:
This study found in a murine model of Parkison's, there is increased colonic permeability.
This study found that chronic psychological stress in rats is associated with increased colonic permeability.
Searching Google I could not find any information about the specific issues, specific clinical symptoms or clinical consequences that the colonic form of intestinal hyperpermeability produces. Perhaps there should be more research on this. Sucralose is used to measure colonic permeability, by the way.
The small intestine is found to be sterile in around one half of North American subjects, and in the other half contains bacteria. Ref:
1 The colon of course always contains large populations of bacteria.
So when you are talking about the inflammatory issues connected to the translocation of bacterial LPS across the intestinal lining, I guess this can happen in the small intestine (in the cases where the small intestine is not sterile) as well as the colon.
The distinction you are making between small intestine hyperpermeability and colonic hyperpermeability is interesting, but I think most of the time when researchers refer to leaky gut or intestinal permeability, the implication is small intestine permeability.