I think this is a viable approach:
Generate a set of keys using your preferred encryption scheme using a random generated passphrase. The trick here is with the passphrase. The key is known, but we will create a time capsule using the passphrase.
Choose a passphrase such that, if we create a salted hash from it, it will take approx "n" years to calculate the passphrase given known salt and hash using today's computing power. If we want to create a 20 year capsule, estimate our computing power 20 years from now, and create a hash that will be one-month computable by either a user or a supercomputer in 20 years, depending on target for the capsule. Figure, for a 20 year time capsule,that it will be decryptable by a megacorp in 15 years, or a user in 20.
Encrypt data using keys with random passphrase, store the key and the hashed passphrase, and don't store the actual passphrase. Now preserve the data and, at some point in the future, you will hopefully have the computing power to recover your data!