So you've opened it before fine? I doubt leaking batteries could have caused corrosion to the point that two blokes with multigrips couldn't open it. There's a product called mouse milk, it's a lubricant that would help with that. Apply it around the seam and let it sit for a few hours so the lube can get into the thread, then have another try.
Yeah wd40 etc probably wont penetrate properly. The mouse milk is a penetrating oil, i'd dare say there would be plenty of other types of this available. If your town has a hardware store you can just try asking them for a penetrating oil and see how you go. If not, try the wd40 (a lot of it) and leave it overnight to give it a chance to get in there.
Would putting it in the freezer for a few hours, then a short application of heat to the outer section (say hot water) and then brute force do any good?
