DJI can't provide you with an AGL height. They absolutely do not have the technical means to do it.
What about using the US map MSL database for elevation? Litchi has access to that in their mission hub. Google does too in their Google Earth app. As long as you have access to that data, I would think it's a simply math calculation to get a relatively accurate AGL estimate when in a hover almost anywhere. I think if Litchi wanted to, they could implement their woman's voice to announce AGL estimates on a regular basis, or when prompted. Wouldn't that be the bees knees?
This data should allow easy AGL calculation to be fairly accurate (within 50', maybe 25') in the app, for very little money, no extra weight, I would think.
1. Access to MSL map elevation data via internet, via cellular.
2. Known MSL elevation from take off point, based on the internet MSL data.
3. Known GPS map location (horizontal) of the craft on a quasi real-time basis, as received by the craft today.
4. Known present relative elevation from take-off point, based on barometer measurements from take off point, as calculated today.
Put all the data together from the above 4 sources and I think you could calculate AGL most anywhere on the map, again...... only IF you have an internet connection.
Lastly, even though GPS elevation isn't accurate, granted, sometimes up to 100' off, vertically, this is better than nothing IMO. It takes a lot of experience to estimate AGL elevation based on the view in my iPad. In some cases, such as hilly or mountain terrain, I would appreciate a GPS estimate from time to time, knowing well it's not very accurate, because it's better than nothing. But again, GPS MSL elevation data, as inaccurate as it is, doesn't do any good without #1 above to calculate AGL, based on map location.
And for those that think the barometer is accurate, it's NOT! I've had my Phantom4 come back after a 10min flight to my cliff launch site, where the VPS sensors are 100' above the ground, and the craft is even with my head, just 30' away from the cliff I took off from. GO would show the craft is 50' high (relative to take-off point), when it should say 6' ! So the barometer isn't very accurate either. Others have reported this anomaly also.