It is a fantastic data set and it would be great to integrate it into GeoNames. A neighbourhood reverse geocoding service based on the data is already available :
http://www.geonames.org/export/#neighbourhood
well, in case they wouldn't accept removing the copyleft, wouldn't it make sense for geonames to change its license by adding a copyleft ?
When you think about it : a copyleft could protect your project and make sure nobody takes the geonames data and releases it as something non free.
Another possibility might be to provide dual licensing. For instance, there could be a 'light' version of geonames available under CC-BY, and the 'full' version available under CC-BY-SA.. That could probably cause confusion, but it's worth thinking about it, isn't it ?