So I’ve got a first pass at smooth transitions between SubRegion patches in place (there are about six subregions visible in this screenshot), and I’ve reached two conclusions:

First, you can get some pretty epic landscapes really easily this way.

Second, it’s way too slow in its current form, and will need some extreme optimisation.  The slow bit is creating an interpolated “Terrain Style” for every terrain vertex which is in a border between subregions… mostly to do with interpolating terrain colors, which is complicated due to the 4D color space with flexible gradients that I’m using.

But I’m sure I’ll think of some optimisations for these calculations.   I’m pretty sure that this is the right direction for me to be going;  just need to solve the tech issues.  :)