I don't claim to know how it is everywhere, but a high-voltage tranmission line crosses my parents' farm. My parents own and farm the land, and the power company simply has an easement that allows them to enter and fix it if necessary. There is no right-of-way. I'm convinced that the hay growing under the line is a far easier investment than solar panels and I see a detriment in putting solar collectors under something. The power company doesn't mow this land, my dad does. They cut the tall trees that may interfere every ten years or so, but otherwise, no maintenance has been necessary. If they did buy the land and placed collectors, they'd have to mow and maintain the land and it might inhibit maintenance of the hi-voltage lines. The reason solar isn't 'taking off' right now is probably not related to lack of land to install cells, more it is that the cells don't generate enough electricity to have a reasonable return on investment. I've heard that it takes decades for a solar pv cell to generate the equivalent energy that it took to produce it. Maybe mirror-farms will do better.

I think it is a great idea but the land owners might not like it unless paid a little for it. They already implemented something like this in California. They leased roof tops form companies and filled the roof with solar cells to help pump power back into the grid.