> Imagine if we had tons of people incentivized to deploy these? We could make "cyber" phased arrays on a massive scale by doing time of arrival analysis and so on.

This is already going on. Some transponders don't report position (often seen on older turboprop airliners in Europe), so feeds from multiple receivers are combined to triangulate them. It's called multilateration (MLAT). https://en.wikipedia.org/wiki/Multilateration