I would presume that, before the sea gates were closed, travel to and from Metru Nui would have been rather common, not only from matoran of other elements but other species as well. Metru Nui was the information capital of the Matoran universe, and just as our head is connected to the rest of our body, I'm sure that Metru Nui would have had interaction with all sorts of foreign beings. Additionally, the implication that turaga dume had the Toa mangai close the sea gates also implies that there was travel through them to begin with. As for Matoran of other elements actually living there, I feel we would have at least heard mention of some of them in some form of media somewhere. As it stands, the possibility is very very slim.