Tinned fish

[Lightspeed]

I can kind of see where your coming from. Love the art work by the way! it could work but is it going to be high maintainence? If it breaks when the wall is in place then you'd be buggered.
I was thinking.
If you have a pipe that is a really tight fit to the tin, then maybe the air that the tin is pushing out of its way on its descent will slow it down naturaly, as the displaced air will only be able to escape around the side of the tight fitting tin?
You'd need to make sure that there was a tin already at the bottom to stop the displaced air from rushing straight out of the bottom and an airtight or near enough door at the base of the pipe.

Does this make any sense?

Petanque Magnet on an extended length of string.... attach to tin, lower tin into tube until at bottom..... sharp pull on string to disengage the magnet :-)

All fine and dandy until the string breaks, then you'll have to empty the tube to recover the magnet and start all over again.

LS