12.3.1 f. says “Add box lines for clarity if the content of the sidebar interrupts the flow of text.” Does this mean we are permitted to add box lines in this instance even if there are none in print?
Yes. And try to do the consistently if the same type of sidebar recurs more than once. This is a significant change from 1997 Formats. I recommend caution and restraint so as not to inadvertently mislead the reader about boxes that aren’t there. However do not add a TN about it either. Intrusive. This is totally about transcriber judgment.