read section d under exclusions. If they feel that mountaineering is extreme..... BASE must be as tame as horseback riding. Unless it's truly written, I'd personally error on the side to talk with them personally than to get whacked and find out your screwed. Just my 3cents.
WorldNomads specifically has BASE Jumping on the list of things they cover under their Explorer insurance. You must be from the USA and you must buy the explorer package.