Thank you for the great question, Christina!
You are correct that the GTM does not provide a dedicated symbol for a line with endpoints over an item. As you mention, you would have to defer to GTM 11.2, which states “If a print symbol is not defined in UEB, it can be represented either using one of the seven transcriber defined print symbols above, or by using the transcriber defined shape symbols in Section 14.”
In this instance, using the first transcriber-defined print symbol (.=?), if you haven’t already used it in the volume, would be the best course of action. Be sure to identify it on the Special Symbols page or at the point of use in the volume as something along the lines of “line with endpoints symbol”. Also remember that this symbol has a grade 2 meaning and would therefore need grade 1 indicators as appropriate.
The braille for your AB with a line with endpoints over it would be as follows:
;;<,,ab>.9?
Note the use of the expression directly above indicator (.=.9) per GTM 7.9, which reads “If something is written directly above or below a term rather than to the right or left, use the directly above indicator or directly below indicator instead of the superscript or subscript indicator. Common modifers [sic] such as the bar, arrow, dot, tilde, hat or arc are treated separately in Section 12.” Since this is not a common modifier, you would need to utilize the directly above indicator to inform the reader that the line with endpoints appears above AB.