Yeah - there’s an embryonic initial beta of Dronepan for Android … but since it’s version 1.0.0 and it’s 15 months old, I don’t think I’ll try it.
I have to say my personal preference is to leave what isn’t part of the initial image files as a solid colour. Even Hangar does the same (emblazed with their name.)
I have thought about taking manual wide-angle photo straight up at the time I’m taking the pano (if it’s overhead) and using that to fill in the sky. But that would only work on a day with crisp/clean cumulous clouds for it to correctly position the top image. Something to play with one day.