I don't think its really the PH change in the soil but more the absorption of nitrogen or micro nutrients while they are still dissolved in the coffee. My friend in South Africa tried coffee on his villosa with amazing results (the soil was live sphagnum). Nobody really knows why this happens, but most think it has something to do with the minerals in the coffee that can be safely absorbed into the roots. After two days, whats left cannot be absorbed which is why you wash it out. Heres a pic to show the differences in leaf size... some plants in the GH haven't been coffee fed in a long time but their massive leaf sizes are still being produced.
<a href="http://s1222.photobucket.com/albums/dd495/Lance_Plater/?action=view¤t=DSC_0055-2.jpg" target="_blank"><img src="http://i1222.photobucket.com/albums/dd495/Lance_Plater/DSC_0055-2.jpg" border="0" alt="Photobucket"></a>
N. spathulata 1 month after feeding... the earlier leafs were 1 inch long while those big ones there are about 5 inches long and a lot fatter (and the basals are vining now)
Yeah, Id recommend the mountain coffee method... just make sure its not hot