Yes cold crashing will work with lager yeast. Why do you think lowering the temp would cause the yeast to become more active? I also believe that 50 is a little low for the San Fran lager yeast (if that is what you are using). Whitelabs recommends 58-65F with WLP810. You might want to do a Diacetyl rest before you crash. Higher temps increase yeast metabolism which will help clean up the Dicetyl which can form from yeast being stressed by a lower ferm temp.