Gotta say I've been enjoying this thread. It seems that just about every New Year's celebration isn't complete without a laser show beamed onto the side of a building somewhere. So it's hard to imagine any serious regs against it. But I'm sure you could find an overzealous D.A. (the initials could stand for more than one term) who'd try to make a case of it.
The Capitol building itself was never beamed by me, it was several degrees to the side of Ann's location. It's not against the law to shine a laser on architecture anyway, is it? Aside from an airport control tower... ;)
Guy's idea about multiple mirrors is interesting, but it would sure take some mighty fine mirrors. Any of the ATM's on the list have an interest in grinding FLAT mirrors? Actually, slightly parabolized might be better, in order to reconverge the beam. Let's see, 20 miles with a six-foot mirror. That's a little over F/17000. Any takers?