In case anyone's interested here's an updated version of the engine:
http://youtu.be/XkSS_veoSg0It has me talking over the top explaining a few things (so apologies in advance..)
It's using a 8K x 8K heightfield and a 128K x 128K texture (effectively. The dynamic loading and voxel down-sampling effectively gives the same effect as a mega-texture).
My L3DT design skills are still pretty basic and I normally don't spend a huge amount of time designing. I tend to just scribble in some mountains, rivers and default climates, then leave it to bake overnight.
But it looks pretty cool seeing a large map with (effectively) a high resolution mega-texture imo.