To the best of my knowledge, there is not a UEB symbol for barred brackets (which look like they have an extra vertical bar right next to the vertical line of the square bracket; aka, "mathematical white square brackets"), shown as ⟦ and ⟧ in print.
Before anything else, we must ask, "Are these brackets the only ones used in the print?" If so, then our job is easy. Just use the regular UEB square bracket and explain the change from print in a transcriber's note at the beginning of each volume. (Braille Formats 2016 §2.6.1.d)
If print uses both regular square brackets and barred brackets, then I believe you will have to use a transcriber-defined symbol for the left barred square bracket and another transcriber-defined symbol for the right barred square bracket. Rules of UEB §3.26 discusses transcriber-defined symbols.
Please let us know if you have follow-up concerns or questions.