In theory, if you were to monitor when enough users are promoted to legendary status, it would be possible to reverse engineer what the secret seed is (you would however need to take into consideration that the required activity for each member may not be divisible by 14), although it would probably take more effort/work then it is really worth.
If you can find the secret seed by looking at when people are promoted, then you've (more-or-less) broken SHA-1's preimage resistance. This has never been done before, so if you do it, you should probably publish a paper about it.
If I have many examples when one out of two variables is known and the other is unknown and the output is always the same, then I would think that it would be possible to determine what the unknown variable is. I would think it would be similar to figuring out the private key that was created using a weak RNG.
I would think that someone would essentially use some level of computing power to "guess"/hash what the secret seed is based on the above known variables.
I was under the understanding this is why bitcoin related gambling sites generally have their server seed expire after (usually) 24 hours and have their 'rolls' result in a number that is 4 digits (usually two before and two after a decimal).
Although after reading a few wikipedia articles, a few stack exchange answers, and the beginning of a paper (it quickly became more advanced then what I could understand so further reading would have been futile), it sounds like my theory is incorrect.