1- In the Bookends User Guide for version 12.7.7, on page 264, in the section "Superscript & subscript", I can read:
In the Bookends Preferences, "Scan and Bib" pane, the symbols that I can choose from the "Superscript character" popup menu are: ^, `, or ~The symbols you may use are:
Superscript: ^, @, or ~
2- There is another problem about those symbols, their meaning about superscript interfere with their meaning in the "Field order" section of the Format Manager. I use often the ^ for conditional groups. The problem is I cannot use the ^ for superscript inside a conditional group.
The same would be true for ` or ~ that has a special meaning in the "Field order" section of the Format Manager.
I suggest offering the option to choose: ´ or ∞.
You could also use _ for superscript and let the user choose another symbol for the other available for subscript.
Thanks for your listening and (I hope) implementing this (and documenting it in the user guide.