An abstract1 presented at this year's 70th Scientific Sessions of the ADA found that supplementing with caffeine prior to exercise prevented low BG:
"Caffeine ingestion prior to exercise reduces the fall in BG, and the requirement for glucose ingestion during exercise in type 1 diabetes mellitus (T1DM), but does not alter respiratory exchange ratio (RER) and thus fuel oxidation. This implies that caffeine augments endogenous glucose production with exercise. Caffeine may be an additional tool to reduce hypoglycaemia during exercise in T1DM."
It was a small study, only 5 participants. They took a 5mg/kg dose of caffeine. That works out to 340mg of caffeine for a 150 lb woman, or 454mg caffeine for a 200 lb man. An 8 ounce cup of regular brewed coffee has about 100mg caffeine.
1 Caffeine Supplementation Reduces Exercise Induced Decline in Blood Glucose and Subsequent Hypoglycaemia in Adults with Type 1 Diabetes (T1DM) Treated with Multiple Daily Insulin Injection (MDI), Abstract ADA 70th Scientific Sessions, June 2010