Here in Germany, to my knowledge it is standard to prescribe the use of towels on the machines. Clothes hold the sweat only partially, shorts and tank tops will always allow to sweat on the seats, and even fully clothed users can leave sweat if they get wet enough. Therefore, naked workout would not really be a problem of hygiene.
But I doubt there will be such an offer, simply lack of demand. Even many of "us", who consider themselves as naturists or at least as interested in nude recreation, are hesitant and have a feeling of awkwardness when they imagine such a setting. Only naturist associations may perhaps think about this. In Berlin, there seems to be a very active association with many offers for sport activities, if they had the necessary financial resources, perhaps they would think about establishing a gym, but this is probably way too expensive for a non commercial association.