First of all, amazing work and great write-up too! In case you weren't aware, someone linked the article on Hacker News
Anyway, you mentioned that you were trying to figure out why this (apparently) wasn't done before and being open to other suggestions. Well, 3D ultrasounds are a thing, but they're used for a very different purpose, namely medical imaging:
Apparently Tom Brown, one of the pioneers of ultrasound imaging, already made strides into 3D ultrasound imaging in 1975!
This is the first thing I found when searching for "3D ultrasound" (on DuckDuckGo) so I'm kind of wondering how this slipped through. It's not quite the same application but it still counts, no?
Anyway, I'm not saying this to discourage you - if anything it hopefully gives some ideas for refining your approach, no?
Keep up the good work!
(also, off-topic, but please consider adding an RSS feed to your blog so I can subscribe to it)