Its not advisable to underplant trees near to the trunk. A bit worried abut the pot though - have you looked up the eventual height and breadth of a Monkey Puzzle tree?
If you want to cover the soil you could use decorative chippings of some sort if you like.

Small, shallow rooted plants won't do any harm. Violas, Sweet Alyssum, Lobelia erinus, and dwarf Marigolds are all candidates. They will probably actually help the tree, since that small a tree in that big a pot is likely to stay too wet all by itself.