Having had trouble opening some of the TGD posted by alpha users, I figured I'd try this again with the update since it appeared to be related (problems with certain nodes and displacements). It is with great pleasure that I can report that the process I described for tiling non-flattened terrains now works with my function masks.
I had also experienced crashes with image map masks when trying to add/subtract two masks. I'm currently running a test render with two high res terrains placed within a low res terrain. One functional mask for each high res terrain and the two masks added together and inverted for the low res terrain. This is looking very promising as it greatly increases the amount of padding that can be done, or allows the creation a path of high res terrains over a long distance mapped onto the planet surface