What's the highest glucose concentration (in mM) anywhere in the human body (tissue, capillaries, tumor microenvironment, etc.)?
Glucose concentration in the blood is a highly regulated biologic variable. From personal laboratory experience, it is very difficult to raise a healthy, non-diabetic individual's blood glucose over about 6.5-7 mM (i.e. 120-130 mg/dl). My best guess at where the highest glucose concentration might be in the body is within the hepatic portal vein that drains the intestines after a high carbohydrate meal. The glucose concentration is probably highest in the prandial state or, in other words, when carbohydrates (i.e. food) is being digested and absorbed by the gastrointestinal tract. Many simple and/or complex carbohydrates are digested to glucose and absorbed through the gastrointestinal tract. Once absorbed the glucose enters the venous mesenteric circulation and then drains into the hepatic portal circulation. This allows high concentrations of glucose and other compounds like amino acids from dietary nutrients to be "buffered" by the liver.

I don't have hard evidence to demonstrate that this is the highest glucose, but I think it's probably a good guess. It is inherently difficult to measure the blood glucose concentration in the portal vein - mostly because it's so difficult to get to that place anatomically during a meal. The dog is probably the most used animal for these types of metabolic studies, because catheters can be placed in the peripheral venous and arterial circulation, as well as in the portal vein and hepatic veins, to measure nutrient uptake by the intestine and nutrient extraction by the liver. 

A number of previous studies by Cherrington and colleagues, who have done some of the most comprehensive work in this field, frequently measure glucose concentration and blood flow rates across the liver. In one such paper that is freely available from the American Journal of Physiology by Abumrad et al (1982) entitled, "Absorption and disposition of a glucose load in the conscious dog" you can see from Figure 1 the average plasma glucose concentration in the portal vein from a group of dogs given an oral glucose load of ~40 grams is somewhere in the 250 mg/dl region (i.e. ~14mM). These were otherwise healthy dogs, and similar to healthy humans I would bet the glucose concentration doesn't rise too much further. As you could imagine though, in a diabetic individual who has high peripheral glucose concentrations to start, the portal vein concentration might be expected to be higher. How much higher is another educated guess - maybe 100-200 mg/dl higher on an extreme end (obviously pathologic).

In a healthy human (i.e. non-diabetic) who undergoes a 75-gram oral glucose tolerance test I can't imagine that the portal glucose would go any higher than 300-400 mg/dl (or, 16.5 to 18 mM). But again - these numbers are just educated guesses. This is probably the highest glucose concentration anywhere in the body. Additionally, in diabetic patients the glucose concentrations would be expected to be slightly higher because circulating glucose may be slightly higher, but probably not by too much (definitely not an order of magnitude).