Putting hash on "static", standalone page would mean they use same seed for every player and disclose it every day/ hour etc.
999dice generates new seed every roll
There is a bitcoin casino that publishes what the next will be with every bet. They have multiple fields stating what the last one was and what the next one is. This could be done anywhere, even if its separate on a per user per roll basis.
999dice clearly does cookie tracking to identify users or you would not be able to have a balance without ever logging in or creating an account. They have 4 cookies, AccountID, LastBetCurrency, Language and SessionID. They also have an indexed DB which I think but have not confirmed is just used for some functions that are exposed.
Because of the AccountId cookie it would not be hard to know who you are talking to and since they use MSSQL they could store what the next would be there or just make it part of the session so if you abandon it its gone. Either way they could make it easier to have a dynamic but displayed next seedHash.
Its all about how the developer chooses to make this information available. If this was not the scam before after this thread I am sure that there will be some that try it.