X Wing: Wedge’s Gamble (1996) by Michael Stackpole shows the rebel alliance using similar tricks during the battle of Coruscant.
I'm far more concerned about the people who own/build stuff like this using it as you suggest (or any of the government pets that they own) than I am about "hackers" doin' such things (although, you're entirely right to have that concern as well, because there's obviously those type of folks out there doin' bad things even with the technology we have already).
50000 60-ft mirrors is about the same area as a single mirror 2.5 miles across. So the area of the mirrors is about the area of a city. You gather as much light as the city itself in regular daytime. If you focused all of that perfectly efficiently onto a city, that city would just look like daytime.
Then again, assuming there's no dispersion or loss, 50.000 times the sun focused on a 60ft patch will likely have some impact. But that's complete fiction.
> in fact there is already no common international agreement about where a nation's boundary extends to in the z-axis. The observed limit is "however high that country can detect and shoot something down"
That's the thing though, if you use that definition then 99.9+% of them are fair game, because _only_ ones around equator at a specific height are at a fixed point. Otherwise they migrate a lot and what country they're over would depend on what second you look.