Its difficult and almost everything is working against you but it can be done. I promised a tutorial and it will be forthcoming but I still have to finish the project, write the damn thing (lol !!) and none of that will happen till I can find a gap in my work schedule.
Take a look through the thread above. The basic idea is a plane highly displaced by two shape shaders. To make a ring you'll need two planes, one for the upper half and one for the lower half.
Place your mountains in above your shape shaders and be aware that slope and height for your surface layers will be pretty hard to get right, slope almost impossible and height just plane hard! You'll be breaking all the rules, doing stuff Terragen was not intended for but the results can be astonishing.
Good luck and be sure to post your results.
Richard
