think this is impossible...because the NON-SHINY on some Legendaries (Zekrom/Reshiram, Victiny, Zoroark and Zorua...well Zorua can be caught with shiny code on, but won´t be shiny) is Hard-Coded by nintendo!
Other Legendary Pokemon (Kyurem, Meloetta, Keldeo, Genosect, Virizon, Cobalion and Terrakion) are shiny, while Thundrus/Tornadus and maybe also Landlos won´t be shiny but can be caught normaly with shiny code on!
I know this sucks.....but think Nintendo made this step to prevent online games from "Cheated" shiny Legendary Pokemon.....but you can either use the wild pokemon modifier or an quick egg code with the Legendaries in dayCare or just use the mark shiny code.....