I'm uncomfortable with substituting unrelated puncutation (a colon) for a change of typeface. I realize this is unusual text and the transcriber must devise, but I always try to avoid tampering with the print. Even if you explain it, you're still altering the print unnecessarily. In this case, I would just use double caps for UNIT. Keep the parentheses, keep the square brackets as in print. You will have followed print and you don't have to explain anything. The braille reader sees the same double caps that the print readers are seeing and I'm sure all the students will ask the teacher what this means! You braille reader is not access the same thing as the print readers. I would just use double caps for UNIT and ignore the subscript position. If absolutely necessary, THAT you can explain in a TN and say the double capped UNIT designations are printed in the subscript position. But I wouldn't even bother with that unless there is some unavoidable significance to it.