Ah I understand the question now. I have keel ‘blocks’ on Mary and there is about an inch behind the blocks as they sit with the keels down. This means the keels have to raise, pretty much, half way before they run the risk of hitting the back of the box. Then there are the stirrups that hold the block back these have some spring. I usually notice this when lowering.
I think my ‘blocks’ are too long and when trying to lower the keels after spending the winter on the trailer meant I need to lower the blocks to below the hull line before lowering the keels otherwise they jam against the back if the blocks.
So I guess if I did try to raise the keels by such significant amount I would run foul of this problem. Using some form of collapsible extension to the block at the rear like a loop of thick rubber would make this safe. Replacing the loop of rubber on a removable block would be easier than anything that attaches to the hull?
Another thought is that these are a wholly detachable part of the boat so could be made of something other than wood that would bend in preference to damaging the keel box. Mine unfortunately (depending on how you see it) are made of a wood heavier than water. So are unlikely to act is such a sacrificial capacity ;-(