Coffee grounds on plants...?

Caffeine exists in coffee plants because the plant constantly produces it. It cannot leech into a plant and then remain there; it will be broken down, even if it enters the tissue. And entering the tissue is tough; it'd have to the waxy cuticle, the cell wall, and the plasma membrane. Even if it manages to survive long enough to enter the roots without being eaten by microbes, the endodermis in the root controls what enters the central stele, which contains the vasculature of the plant. The selective cells wouldn't allow caffeine to p through; only mineral salts, water, and other essential nutrients are allowed in, and the casparian strips (barriers of waxy suberin) prevent anything from entering the stele via apoplastic pathways (going between the cells). Think of the cells as bricks, with gates to control what pes through the wall, and the strips as the mortar between them. And if caffeine can't get into the stele then it can't get into the xylem, which transports water and nutrients to other areas of the plant, including the leaves. Your basil and tomatoes are safe =)