That is fairly trivial. In fact, NMS can do that without issues. I’m not even sure whether you need to mod it, I kind of remember it being a mere config parameter to enable “build everywhere”.
The first problem you’ll face, though, is multiplayer. Currently such bases are not sharable. This is not that much of an issue now that they had the time to redesign the online stuff.
The larger problem, I’d imagine, is that your base ends up in Nirvana every time the terrain changes.
Now, people who have seen the latest GDC talk from HG (which is not yet online, unfortunately) said that it was about how they restructured the generation so they could make more targeted changes without screwing over the entire universe every time. This is non-trivial, but there are ways to do it. They’re just often less performant, and they do still limit the content you can add without changing existing things. For example, adding a planetary biome will necessarily mean some planets will change. Though you can structure it in a way that the biome changes without the terrain, but quite often the two are loosely connected.
My guess would be that HG has added enough stuff that they’re confident they don’t have to blow the universe up again with the next update, and that they might enable free base building because they don’t expect to have to walz all over your base anytime soon.