Doing things right at the design stage is always easier than doing them afterwards. Buying a jacket with pit zips and selling (or returning, if you haven't worn it) the one you've got would probably be the easiest, and might be cheapest.
I've seen some of the repair work Penny (Specialty Outdoors) has done. I have no doubt that a pit zip she added would look like it was supposed to be there. Doing it the way that's mentioned in the other thread will work just fine on coated fabric, and is not that hard. Probably would be a good idea to practice, if you're not an experienced sewer. You might also get a dry cleaner to do it. They're unlikely to have a waterproof zip or seal seal tape, though.