[croikle]

It's a cute idea, but you have 1/r^4 intensity scaling, which is prohibitive even at lunar distance. The lunar ranging experiments receive "one photon every few seconds" [1]. You could probably do better with bigger equipment, higher power, and perhaps an orbiting base station to skip the atmosphere, but the numbers are going to win before too long.

An active repeater would give you 1/r^2, though. For example, you could ping Voyager I for a 36-hour delay. However, that's likely not feasible at light-year distances, let alone the problem of getting the devices there. :)

[1] http://www.jpl.nasa.gov/news/news.php?feature=605