Currently, it's only possible to align ballons horizontally or vertically.
Since I almost always attach my balloons to an isometric view it would be great to have the option to align them in a isometric manner, or to select an edge to align with.
I found this quite old topic about it: http://autodesk.lithium.com/t5/Autodesk-Inventor/Balloons-on-an-angle/td-p/2969578