I understand wanting to the judges be impartial as possible, and not have high ratings influence their thinking. That said, 655 people entered this contest and 635 of them who all worked really hard are kept waiting for something they're not going to get. For everyone else, the rankings and ratings will be the only way (aside from comments) to know what they did well and what they can improve on in the future. If you're already able to hide the scores and rankings of the top 20 from the judges, perhaps it would be better to just instruct them not to look at the general contest results? They already know the OST's they're evaluating are in the top 20 anyway- maybe it's okay if they do see it, since their evaluations don't affect the rankings anyway?
Either way, I know it's a lot of work to host these jams and I still really appreciate what you're doing! Just something to consider for the next one.