That sounds like something I'd be interested in keeping tabs on as your progress continues. I wish that I had ANY useful coding talents to speak of, I'd offer to help. I wonder if there isn't anything that can take an existing planet map at lower resolution, and then use a given segment of it (of known dimensions) to seed a sort of 'refinement' randomization which would fill in the gaps, meaning the master map wouldn't generate consistent output for any given patch you wanted a closer view of, but as long as you saved whatever render you decided to use, that shouldn't be a big deal. You could open the closer-area image that you saved from a previous rendering. The rest of the planet would literally just be there as scenery. (I.E. no 20GB files to keep track of.)

I'll check out L3DT.