I have done it for an occasional long stitch in FMQ. I used one strand of matching thread on a needle with no knot on the end. I put the needle up from the bottom to come out right next to the long stitch on the top, hold onto the thread end so you don't pull the whole thing through. Put the needle tip back down on the other side of the long stitch and pull it through to the back still holding on to the thread end. You now have the 2 loose ends on the back. Knot them together OVER the back thread and using a self threading needle, bury the loose ends into the sandwich right next to the back stitch and give a sharp tug to bury the knot. Clip off any extra thread and the long stitch is fixed.
I would post a link but so far I have not seen any other obsessive, compulsive person that has to fix their long stitches. I no longer have this problem since my Bernina with the BSR.