If only there were a series of mirrors, each N light-years away, you could blast a one-time pad out to the mirror of your choice and then announce the date the reflection is expected to arrive.
Nice idea IF we had such mirrors. Afaik the farthest away is on the moon.[0]
Also a one-time pad needs to be as large or larger than the data used to encrypt it.[1] So your plan cannot be used for the several gigabytes wikileaks insurance file. But perhaps you can "pad" the key used to encrypt it? Probably.
It'd probably be easier to design an ultra-long delay line ASIC that internally maintains a state for a certain count of clock cycles, and spits it out after a while. It might still be possible to do a cryogenic attack, but that wouldn't be very easy to do.
Or maybe some sort of superconductor/magnetic state device.
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. :)
The use of black holes here could work, if you had an accurate enough representation and or a large enough receiver dish. Fire your one time pat just in front of a moving black hole and have it circle back through the gravity well before the black hole moved on.
