I am sure it would be fine, as long as you maintain line-of-sight so you don't lose signal. You can fly lower than the take-off height. Yesterday I was practising orbiting a tree in a field at about waist height but the height indicator said -1 metre because it was lower than the take-off height. I have flown down a high sea cliff with my DJI Spark and that handles height in the same way. I don't remember looking at the height indicator, I had enough to be concentrating on! Flying over then down a cliff is exhilarating. Go for it! I certainly plan on flying over and down cliffs in the future. If a drone is below its take-off height and the signal is lost it will rise up to the RTH height before returning home, but you would almost certainly be wanting to cancel that as soon as you regain control so you can carry on with what you planned to do.
I don't think you can do a flight plan with a negative altitude, at least not without editing the files. That kind of experiment is for people who want to make everything a challenge. Standing on top of a cliff and flying a drone is an amazing experience and the footage is usually pretty awesome. Having the ability to look up as well as down means that my Spark will not be my travel drone going forward, it has to be the Anafi.