One solution might be to have the password sent to the registered player's email address 5 mins before the start of the tournament and have an "informal auditor" that is chosen and changed on a weekly basis that receives an email with all opted-in player names and if a player is in the tournament that is not in the mail, he gets flagged and action taken against.

Another option might be that the "price" for the tournament is slightly increased to cover costs and players are sent a text message to their registered mobile numbers with the password. Not sure how viable this is with regards to mobile carriers world-wide, but it's just an idea for an ideal world (if there is such a thing).