I believe discharge planning would qualify as an acceptable position to sit for the exam. Go on the CCM site and review the qualifications. It's pretty self explanatory and will answer all your questions. I believe that you have to work under a CM certified supervisor, and you have to have done discharge planning for a certain number of years.
I obtained my CCM certification and all I ever did was UM. So I am pretty sure you would qualify as long as you fit the parameters explained on the CCM website.
I found that CCM certification was not required in a lot of positions I have taken, so I let my certification lapse. I will have to sit again if my job ever changes and am required to have certification. I have not found the need to do so as of yet.