I'm not sure that a zoom climb has much of a role in flight testing, but I have certainly explored the characteristic in some more sleek GA types. You can certainly achieve a rate of climb well in excess of normal powered performance by applying some inertia, but it takes a touch to make the most of it without incurring excess drag as well, very easy on the pitch change, and maximize it off the top. If you come off the top too late/slow, you'll throw away some of the altitude, which you cannot maintain. I remember doing ultimate ceiling testing in a normally aspirated, carburetted Cessna 185. With terribly limited climb performance, I could zoom over 21,000 feet, but I could not hold the plane up there, it would settle back. The highest I could sustain was 20,800 feet - maximum power 12" MP, stall horn steady....