I was able to add subscript to a balloon. Give this a try and let me know if it meets your need.
In an ipt, I went into iproperties to the custom tab and added a field I titled "SUBSCRIPT". However, this can also be done to any other Inventor component filetype.
In an idw template, I went into Style Manager and added a Bubble style I called Balloon 2-Line. I then went over the the Balloon Formatting field and chose Circular - 2 Entries. By default, the Property Display is "ITEM,QTY". However, that can be changed by clicking on the Property Chooser button. In Property Chooser, create New Property "SUBSCRIPT", add "SUBSCRIPT", and Remove "QTY".


Now, whatever you wish to have in that second line must be entered in the ipt's custom field named "SUBSCRIPT".
