Not sure I fully understand the question. Is it whether using extracts will work as the sole source of flavors for a cider or whether you might add some extract to a cider (hard) to enhance the flavor? Cannot say that I have ever tried to make a wine (or cider) using ONLY extract but I have certainly used extracts to make liqueurs and have added extracts to meads (and wines) to increase complexity of flavors. indeed, I am currently planning on making a nut flavored mead with the addition of extracts: very hard, in my opinion, to extract pistachio, hazelnut, pecan and almond using water as the solvent, and using alcohol as the solvent produces what is called an extract!
All that said, could you use extracts as the SOLE source of flavor for a cider or apple wine? I don't know. Given the price of a bottle of extract and the possible loss of a gallon of wine that you could write off as a sacrificial experiment your question, kwiley, is at once quite reasonable and quite easily and inexpensively open to an empirical test. I wouldn't sacrifice 5 or more gallons but a single gallon... go for it.