I quite like the results that some of these flat-world tectonics programs produce, like Nortantis and WorldEngine, but they do fall down because they operate on a finite flat plane instead of a sphere - or rather, since they roll over opposite pairs of edges, they work on a torus. It occurs to me that one way of partly dealing with this is to consider a spherical cube, i.e. a cube whose six faces are projected onto the surface of a sphere, and to generate a "world" which is four times as wide as it is high; e.g. 4096 x 1024. Map that to four consecutive faces of the cubic sphere, either around the equator or around both poles; you are then in the business of merely filling in the other two faces, one of which will possibly be ocean; you might also have to move some bits around so that the bits that wrapped around over the long edges of the map are in sensible places.