Improvisation is admirable, but tying someone into a line goes against the basic principles of river rescue. You may be ready to cut the line on your end, then your buddy gets to swim while trolling line. A much better solution would be to use a pfd with releasable ring or Mongo used to produce one that would go over any pfd. That way you can release without having to take the rope with you. Another option is to use a throw belt that has been positioned around the chest, you attach the rope to the belt and if need be you release the entire belt.