Is the block timestamp reliable enough to timestamp the hash of a document?
A hard fork, which is essentially a blockchain split, would result in the current blockchain to be declared as deprecated and worst of all; not verifiable in the future by the new/upgraded software which will abide by different rules and will not be able to accept pre-fork blocks and their transactions as valid, so if the above scenario would come in life then you would need to legally prove that some contract existed a few years ago in a blockchain that is now deprecated and off-line, whose value would not be much different than this of digitally signed plain-old text document. As a consequence, to prove that smart contract which was associated with a certain tx in the old-chain was signed before a certain time would be even more difficult. 

It's also important the precision of time you're after. If a few seconds or even few minutes could invalidate your claim then this will not work for you as geographically distant nodes will receive blocks at different micro-times, as it happens with all large-scale p2p networks.