The person placing the bets can simply agree to only place N bets per week, and never 'randomize' the seeds. Then each week he publishes his N betids, and all participants can look up the rolls on the site and see that the seed hash is the same as it should be, and that the 'nonce' values are consecutive.
If there are any gaps in the nonce sequence then he's probably hiding a big win from you.
You could be the patron saint of the working class, by giving them a fair game without concerns about cheating colleagues or having half their money stolen by rapacious lottery operators. On the other hand you would probably get blamed for bitcoin fluctuations, general concerns about integrity with online gaming, etc.
Do you have any estimate on how many coins Nawaka brought to the table. I can see a lot of difficulty making that calculation, but I was curious. Did he bring 10K coins to the table to win 10K coins?