Hi Jan, I can’t honestly say if soft soldering the barrel wall would be strong enough, is this your own clock or a customers? If it is your own clock then you could try it and see what happens, worse case scenario would be if the spring broke it would open up the barrel wll crack further and I would absolutely do what William suggests and move the barrel wall hook, plugging the old hole whichever route you take, this is something I missed out and William was good enough to pick up 
Soldering brass is no different to soldering any other metal, you just need to make sure it is hot enough for the solder to adhere to it. Make everything as clean as possible (glass scratch brush or wet and dry paper) and a good flux helps.
Paul.
