You might enjoy this radio program broadcast on Australian Radio National in 2009 when the GFC was more topical.

http://www.abc.net.au/rn/backgroundbriefing/stories/2009/2526727.htm

The program examines the change in cluture from management as an apprenticeship to the advent of the MBA.

Why would tracking be slow? Parallax being what it is? As long as the mirror is tracking targets within the same quadrant of the sky, it should be able to switch fast enough.

As for modulation, I think you are making unstated assumptions about reflectivity and switching speeds. How are you imagining the focus to be shifted and reflectivity varied? Mylar held by variable stays, held taught by photion pressure, for example? Easy, point a big dish at a little dish for a two reflector system, and your traverse speeds (governed by mass) and modulation speed (also, to some extent governed by mass, depending on the method) are dramatically reduced, as now you only must move the smaller reflector.

I know very well about solar concentration with mirrors. A reflector array in the L1 would have the potential to extend the day cycle of a solar collection site.

http://www.sbp.de/en/html/solar/dish-stirling.html

It all depends on the size of the reflector, or better, array of reflectors.

It is probably more effective (given rotation times etc of a very large sail) to aim the array at an orbit and impart sunlight to objects as they approach in that vector. It may well be practical to rapidly turn on and off the reflectivity of the sail with shutters or LCD cells or the like, avoiding striking unintended targets.

The ability to de-orbit any satellites equipped with a solar sail alone might make this worthwhile. Also, point it at a terrestrial solar farm for double the energy production!

Outside the reams of the achievable right now, to be sure, but what are we without our dreams?

I wonder at the effectiveness of putting a very large focusable solar reflector in a high orbit, perhaps at LaGrange point 1. Such a solar sail could be used to give thrust to satellites equipped with a sail, or even large bits of space junk. Obviously it wouldn't give much Delta V to junk, but it might give some, and it would be essentially free. Junk in high orbits takes hundreds or thousands of years to de-orbit, and any means of reducing the velocity of said junk would drastically reduce that time. Additionally, with a variable focus the mirror might be pointed at solar cells of existing satellites, which could improve the thrust gained from Ion Drives, assuming enough reaction mass remains to take advantage of the extra watts.

Idle musings. Feel free to shoot me down.