I found a plastic back [http://www.bhphotovideo.com/c/search...ialSearch=yes] that I attach to the back of the Hasselblad film magazine to hold the slide when I remove it from the camera. No matter how careful I was with dark slides I would loose them for day or weeks at a time. Perhaps you can use the same device on the Bronica SQ.
Interestingly, my Bronica ECTL had such a feature built in to the film backs. I recently acquired an ETRs system (which I love, by the way) but was surprised to find that it does not. Progress, I guess.