Hello Andy,
if a True Type font has been uploaded with transaction SE73, then it can be used as a regular font in the form, and as such, the font height can be adjusted to change the output of the barcode (which now will be actually represented by the uploaded True Type font).
Also, if the uploaded True Type font does not offer a solution for the Bearer Bar, you can try to create a box (table?) around the barcode's place and use it as Bearer Bar.
Best regards,
Laszlo