high relative humidity will not help much in this case because the dry substrate sucks the moistrue out of the eggs.
do not put a heater directly under the eggs because they may get too warm. high temps kill a clutch faster than cold temps.
personally, i would make an emergency incubator and pull them out asap.....they may make it full term if left with mom, but run the risk of getting dead in eggs because the babies can't get out of the dry hard shells...