Ruth Rozen, our string expert, sent me a copy from a Suzuki cello book where she is going to meet the same situation. A footnote there says "Closed hand position is marked with O. Open hand position is marked with X." The Xs and Os in this one appear between notes, not adjacent to the finger number. I recommend that you and Ruth discuss this challenge off-list and let us know what you can suggest. If you don't have Ruth's contact info, I'll give it to you off-list.
My thought, as a starting point, would be to treat these markings as word-sign expressions, explained on the TN page. Where they occur between notes, that is where I would put them in the braille. Under the music braille rules, they cannot come between the note and the fingering, so if the X is given with the fingering 4 in your violin book, I'd put the word-sign X before the note. A TN is necessary in any case.