Good day, Susan!
The attachment "Nemeth omission fields" is too big for our website to upload. Still, I think I can see from the UEB attachment what you're asking about.
I agree that there is not a code rule that addresses this issue specifically. So, we'll have to just talk about practical solutions that do not violate any codes.
One way you might indicate, in UEB and in Nemeth Code, blanks in which the student is to braille their answer directly is with a tactile line (or any tactile shape, since tactile shapes are not part of any code). Or perhaps you could "draw" a box with braille cells (I've heard this referred to as a "P-box").
Alternatively, *perhaps* you could insert a heading, enclosed in TN indicators, before the space left empty for an answer. That transcriber-inserted heading --or maybe a regular 7-5 TN-- would always be in UEB and could always say the same thing (e.g., write your answer below).
I hope some part of the above helps. If you have time, please post here a description of what you decide to do.