You're missing the distinction between local and global blood glucose. A local low is when one particular area of the brain has used up its supply and has to wait for blood to circulate to replenish it. To prevent that from happening, you'd have to either increase the rate of circulation (ie, exercise in a way that increases heart rate), or increase the global blood glucose concentration to a level that's too high for the idle portions of the brain. A global low is something most people never experience, since the body stores and releases sugar to keep it from happening.
(I am a type 1 diabetic with various fancy equipment for tracking my blood sugar. I have personal experience as to what various blood sugar concentrations feel like, but they may not be representative of more typical biochemistries.)
If only there was some sort of fluid circulating in the body and providing nutrition to every cells which needs it... oh wait...
If you look at the experiments, like the one with the dogs linked above, a plain sugar drink at the right time improves willpower. These tests were all done on people and animals without diabetes, I can easily believe it won't work for you.
There's considerable amount of evidence that willpower is severely diminished if blood glucose get down, and this effect is not limited to humans. And a small sugary drink at the right time is enough to restore it.
We're talking really small numbers. Total blood glucose of a healthy adult is about 5g and it varies within fairly limited range. Then there's maybe 45g in total body waters. Then there's about 100g of glycogen in liver, plus yet larger amount in muscles and other organs, but which doesn't seem to take part in sugar level regulation. For comparison a small can of coke contains 33g - a really small amounts at appropriate times can make a big difference.
This leads to two issues. First, is blood glucose a good explanation for willpower deficiency and therefore akrasia? I'd say there's significant amount of evidence that some effect exists, but is it really the most important factor? Humans are complicated, science knows very little about how we work, and probably half of what it "knows" is false or at best only half-true. Caution is definitely warranted.
And the second issue - if this theory was true - and by manipulating blood glucose levels you could achieve far greater willpower whenever you wanted, what would you do? It seems that exploiting it isn't that easy, and I'd love to hear if any of you tried it before.