I think it is not the patent only
It is more the technically difficulty and know how about materials what makes the Schlumpf Hub special.
So, if you want to start building Schlumpf Hubs, the best way is, to contact Florian Schlumpf.
Oh clearly manufacturing one is non-trivial, but up until the expiration of the patents it would have been difficult for anyone to even consider designing & manufacturing a new multi-gear unicycle hub without risk of patent infringement.
At the price that people are clearly prepared to pay (well in excess of $1000 for a used hub), and the explosion of interest in unicycling, perhaps now a company might start thinking about it.
Or perhaps Florian will have success soon with finding a company who they can partner with to produce future hubs.