although, i have never fished a mouse fly for any fish other than bass, i have heard of monster rainbows attacking them in the bristol bay region up in alaska. i assume the flies were worked slowly in eddies, and or slack water, close to shore.
the other mousing technique that i have tried, is for large mouth bass. cast it parallel to the shore and slow twitch retrieve it. caught a fair number with this method and would guess that this would work for pike as well