Why don't you just use as a salt for each roll a random # generated by
http://www.random.org/ or another true random generator site (this one uses atmospheric noise I believe)? Wouldn't that close any loophole for the operator or someone who has compromised the site and can see the server seeds to cheat?
I'm not familiar with everything random.org has to offer.
But is it probably fair? Is there any way I can prove to the player that the seed that made them lose was fairly chosen by random.org, and not carefully calculated by JD to make them lose?