Finally managed to get the next bit going (took ages):
So I can now subdivide and displace the voronoi cell edges to get that fractal coastline look.
More importantly, I can specify a focal point and a falloff range and as edges get further away from it they subdivide less.
That means I can pile on the detail near the player without having to worry about that being replicated across the entire planet surface and basically instantiating a black hole in UE4s memory usage.