Hi there I realise this is an semi-old thread, but I have found a solution to this (also commented it on someone elses thread who had a similar question).
Instead of trying to apply AC: +1d20 to the defender, give the attacker ATK: -1d20.
Obviously you still need to give everyone a -10 to their AC to remove the base 10.
But yeah the math works out the same, and fantasy grounds is built to handle it.