Originally I attached my jib sheet lead blocks with a nylon strap as you are doing. I noticed no tendency for them to slip aftwards. You might try attaching a shock cord or small line to the block to hold it vertical and tie that line to the top lifeline ahead of the lifeline stanchion. It might keep the blocks from slipping aftwards.
Another solution can be seen in the photos of FC Dora Rose on my Falmouth cutter website: [homepage.mac.com
Still another solution is to attach a padeye to the bulwarks at the same location where you are now placing the blocks. Then attach the blocks to the padeyes with shackles. This was done on FC #36 Olive Oyle.
editor: FC News