If I slice the radishes paper thin, is it still necessary to pre-roast them? Or can I just put them on the raw dough and maybe bake it for another...

My initial answer is yes. But I'm just thinking of the variables; Will they burn? Will they release too much liquid and create "soft spots" on the crust? And if you decide to bake it for an extra minute (or two), the dough will overcook...
But I say give it a go. These are just my little anxiety provoked wonderings.