There's a number of hotkeys for navigating the tree, including Parent Bone: ctrl + UP to select the parent bone:
Parent: ctrl + shift + UP
Child: ctrl + shift + DOWN
Previous Sibling: ctrl + shift + LEFT
Next Sibling: ctrl + shift + RIGHT
Expand/Collapse: ctrl + shift + ENTER
Parent Bone: ctrl + UP
Child Bone: ctrl + DOWN
Previous Sibling Bone: ctrl + LEFT
Next Sibling Bone: ctrl + RIGHT
Tree Up: alt + UP
Tree Down: alt + DOWN
Collapse: alt + LEFT
Expand: alt + RIGHT
Is that sufficient? There's currently no button in the UI, but then you could just click the parent instead of a button.
