Will barometric data from my iPhone be used for a workout created on an Apple Watch without a barometer?
Yes, it already works with watchOS 3 and iPhones with built-in barometers (iPhone 6, 6 Plus, 6s, 6s Plus, 7, 7 Plus, 8, 8 Plus, X) running iOS 10.x or newer.

Start an outdoor walk workout and walk uphill. Once you stop the workout you will see the elevation gain on the Apple Watch and in your iPhone's Activity.app.

Example: