I don't think this should be a concern, unless you are diabetic and taking insulin. In that case, you should carry something with you to bring your blood sugar levels back to normal fairly quickly. The easiest thing is glucose tablets. If you have taken too much insulin for your level of activity, then yes, you can "bonk" and since you still have excess insulin in your bloodstream that continues to lower your blood sugar levels after the bonk, you can be in a dangerous situation. The link below discusses this specific situation (it's a little more than a "bonk"... but that's how it starts for a diabetic on insulin)

http://www.essortment.com/all/insulinshock_rjma.htm

Even for a diabetic, it's pretty easy to prevent a bonk or insulin shock/diabetic coma on a long bike ride.