Have you ever seen a Telford unicycle? They were intentionally built with rake in the seatpost. This was so the seatpost would be angled back like a bike seatpost so that the Thudbuster style seatpost shocks would work better. The strange frame geometry did not make the unicycle feel different. All that matters is that the seat be somehow fixed above the wheel by a frame. What the frame does below the seat does not matter as long as the frame stays out of the way. When you get on the uni the seat will be directly above the hub just like a normal unicycle.
So don’t worry about the rake unless you don’t like it because it makes the unicycle look funny.
The Telford’s are no longer made so you can’t get one any more unless you can find one used.
Here’s a picture of the Telford
