I have an Elliptical which uses iFit as well. The heart rate monitor on the elliptical is woefully inaccurate. I previously had a New balance Heart Rate Monitor which was way more accurate, and would give me a better idea of my Heart Rate during the workout, although I had to hold my fingers on sensors to do the Heart Rate reading.
When you start the "Workout" app on the Apple Watch, it takes your Heart Rate more frequently, and I have found it to be way more accurate. I have my Elliptical set to the highest incline at all times, and you are right, it does not register that, however it does calculate my Active Calories based on my Heart Rate, which is way more accurate than the "Projected" active calories on my Elliptical.
I am not a developer at all, but my understanding is that there is a Developers Kit that Apple makes available for the Health App and the Apple Watch, and third party developers are able to allow their devices to read and write data to the Health app if it is supported. I purchased my Elliptical back in 2009, so I am quite sure that there will be no update to allow this to work. I have taken to completely ignoring the information on the dash of the elliptical as the Apple Watch is more accurate. I can easily block the Elliptical data by placing my iPad in front of it, and watching Netflix or listening to music while I work out.