Orson, I would be interested in what solder resist will allow the clearances to be reduced, we have always had to apply a conformal coating to the board AFTER it has been assembled, as you also have to cater for component pins as these are exposed metal. Solder resist per se is put on before assembly and as far as I remember with the SELV (low voltage directives) for europe and USA, solder resist has to be discounted when calculating creepage and clearance gaps. Using a solder resist in this fashion means your pads would have to provide the clearance from any exposed component metal!!!!!!!
Regarding clearances from copper to copper, if a manufacturer cannot make what my design requires in this day and age, with the component requirements, then I will go to one that will. With some of todays bottom terminated components requiring both 'copper defined' and 'solder mask defined' pads, you have to use the same design rules for ALL conductive copper shapes.