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It appears the answer to the original question is finally available now, a decade later, due to Andrej Bauer (et al), who had already provided a helpful response here ten years ago. Though we do not have such a surjection in the effective topos per se, we can use a modification of the effective topos concept to get a nontrivial topos with a surjection from the naturals to the Dedekind reals. See https://www.youtube.com/watch?v=4CBFUojXoq4.