Shh… don’t tell anyone but I already have access to a URC Muni XS frame which will allow for a maximum tyre size of 20" x 4.0 (in theory it is one of my daughter’s unis but there is little stopping me from using it). This frame will take an ISIS hub and has a disc tab for a brake. 
The pink unicycle on the far right in this image. Currently with a far smaller tyre.
And before you ask, I have zero problems with the idea of cycling a pink unicycle. In fact I think it looks awesome!