Charles, all my plumeria are in the ground but they really do benefit from a little planting at the base. A perfect companion I have found is Crown of Thorns since it likes the same conditions and will stay small(if you get the dwarf variety which is most commonly sold). I have also used the variegated New Zealand flax to underplant the larger plants (the flax will get about 20-24 inches tall).
In a pot it might also be nice to put an ivy of some kind since it will trail over the edges.
Let me know what else you find works, I am willing to experiment with some other plants!!