I have cleaned and oiled a SACHS dual drive hub (predecessor to SRAM?). It seemed to be a pretty straight forward process.

However, I have a SRAM Dual Drive that needs an overhaul, and as far as I can tell, SRAM only sells the complete hub, or the complete internal mechanism (plus the click-box). Individual subcomponents don't seem to be readily available.

Shimano seems to have a pretty complete parts breakdown for their hubs, and hopefully one can still acquire the parts, although there is no guarantee that Shimano will continue to support their current product line fifty years into the future, and finding someone with warehouse stock can even be difficult.