+1
I've wanted this for ages. Seen it posted lots of times.
Pretty sure they won't let us do it any time soon, though.
The problem is, people would try to use it to mark minor surface POIs for coming back to later. Meteorites, barrel stashes, crashed fighters, probes, beacons, escape pods, all that kinda thing. And these objects ain't persistent. They won't be there when you get back.
While some might want to mark certain craters or scenic viewpoints or whatever, the most intuitive, obvious use-case, that most people will immediately try to use it for, simply won't work. It would immediately draw huge attention to a big fat limitation of the game, which is currently obscured most of the time by the sheer scale of everything.
So it's not as simple as the game saving a planet name and a set of coordinates somewhere. FDev have this huge restriction to overcome before it's reasonable to implement. I have no idea how hard it would be to make every interesting bit of rock and detritus persistent, but I bet it's a lot more work than just the marker bit.
It's something I would really, really love for them to do, though.