Chrisian nations? Bah! We want a higher colonist gain than 0. How come Spain don't get any colonists on the basis that they believe Jesus was a prophet? That is the game simulating geopolitics that do not exist in our game.
Muslims still have a stranglehold on the Silk Road last I checked, same as OTL. Just recently confirmed by Fatimid annexation of the only Mediterranean Silk road terminus previously in Christian hands, ex-Roman Syria, Fatimids now control all the traditional Mediterranean ends of the land route just as Turks did in OTL. So the Christians still have the motive to explore sea routes to Asia that is lacking for the Muslims. It's not that massively different from the OTL setup, is it? Only that Muslim Spain wouldn't have the massive incentive to find a route around that Christian Spain did, since Muslim spain is best friends with the Silk Road route holder instead of worst enemies.
But why would you have to mod? Couldn't you change colonists in the savegame?