Answer: I'm not aware of any past experiments like this one. It does seem like an interesting idea to research--though I wonder if there are any "real world" applications of this. Who knows, maybe there's a greenhouse somewhere that treats all its seedlings with coffee!
Substances like caffeine (and the "heat" in hot peppers, and nicotine, etc.) are usually considered to be adaptations to ward off predators. These predators can be herbivores like grazing deer, or insects that much on the leaves. (They don't deter us humans, though, do they?!) So unless your plants will have some predators to ward off, I'm not sure how or if caffeine will affect germinating seeds. Good Luck!
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