Personally i feel this is an excellent idea, why use hg/ss for johto when you can use a rom hack? it's genius!
with the inclusion of all the games up to sun and moon it will help diversify the comp scene or something(THINK OF ALL THE NEW AFK SPOTS!!!), and I'm sure it will be easy to code so i imagine if they started now the devs could be done by tomorrow.
as a curious side note if we need both X and Y and Sun and Moon wouldn't we need Ruby and Sapphire and Diamond and Pearl? what about black and white 2 or ultra sun and ultra moon?