Hey, good questions.
I dunno if you've seen, but all these wall/cube placement features are likely to come to the official meta SDK. They've been showing off a few clips, and spatial anchors already exist in the unity dev package. Might be worth just waiting a bit to see what comes in the next few SDK versions..
There's another "map your house" package out there that has the kind of SDK you're looking for. There's a zombies in your home, virtual animal safari thing, floor is lava game. All of them use this other mapping SDK. Sorry I can't provide a better link, am on mobile at the moment.
Your best bet is to check out the recent tutorial from skarred ghost on the spatial anchors, try to get it working in unity, then make your own wall/geometry setup based on that, maybe integrate the official features when they get introduced. My mapper code is rather hacky and fragile, and has strange hooks into all the multiplayer/voice chat stuff. It's hard to offer it as a modifiable package to other devs.
Regarding other platforms/devices I'm desperate to get my hands on some new tech. Yes please.