So outside the consumable WEPs (Water injection, methanol water injection etc), WEP is nothing more than a higher boost setting. Higher boost setting meant you built engine temp up faster, and THAT was the limiting factor. It didn't automatically cut off, you could blow your engine up if you kept it boosted...but if the difference was sucking on the end of a 20mm cannon or maybe melting a cylinder....guess what most chose.
So essentially, cooling off the engine should allow for WEP again. The other part of this equation is the fuel flow needs to go up with WEP as well....(excepting the injectables where the water actually displaces the gas, and they use LESS fuel then at Mil power).