Sorry guys. Have to put the coffee thing to bed. Caffeine, theobromine, and several other secondary metabolites of Coffee are allelopathic to other plants. These are found in the seeds as well as leaves, which inhibit seed germination and root growth in many other plants. Whether this is specifically active upon Nepenthes is up for debate.
However, I suspect that you will find as the concentration of caffeine and other compounds rises, growth of your plants is inhibited.