I would expect the acidity would be extreme and destroy the root system in a short time. You have to remember that coffee, applied as a "fertilizer" to Nepenthes is done sparingly, infrequently, and as a very dilute solution. Planting directly in a medium containing a significant portion of coffee grounds would likely be quite toxic. However, if you can determine the actual acidity (use a meter!) of the proposed mix, and determine the conductivity of the runoff (it must be under 24 microsiemens) then you just might be able to engineer a suitable mix. I would expect, however, that the percentage of coffee grounds used would have to be low (under 20%, or even far less) and you would have to wash the grounds repeatedly in rainwater/RO water before use. (Just like you have to "wash" peat before using it in a Sarracenia mix: right out of the bag, it contains too much salt)