This is a very cool idea! I think this actually should be split into its own topic, for example “Idea for a geared unicycle”. As far as I can see from a quick search, similar topics are quite old. What do you think @Canapin ? Edit: I didn’t see that this olso exists in the current projects. Maybe my reply should be moved there?
For example: https://unicyclist.com/t/geared-unicycles-an-easer-way/80721/2
I have also been asking myself why we can’t just make a geared uni as simple as a normal geared bike. And I bet a lot of others have as well. I hardly have any practical knowledge about this, but most examples of custom chain geared unis I have seen are for some reason far more complicated than that:
I also notice that both George Barnes’ geared uni and the Huni Rex has the chains running down on both sides. I don’t know why one chain should not be enough, like you suggest.
Some practical limitations i do see: You will obviously get a higher Q-factor.
You will, as you point out, need to find room for the “machinery”. And if you need a bigger frame than a 20" frame, why use a 20" wheel, and not a bigger wheel instead?
Also, if the machinery is on the outside of the frame, you risk damaging it when dropping the uni. It really doesn’t take much abuse to damage these parts even on a bike.