The rag will stop the balls and spring from shooting out. When you pull the spool out you will loose oil out the hole. As sterlclan has pointed out a few times that using a shop vacuum in the oil reserve tank will limit the oil that is lost. Its not something you can simply hook up and walk away from though. It sounds like you simply need new quad rings that seal the ends of the spool. Get a kit, remove the spool and change the quad rings on the front and back of the spool. If you do loose the balls and spring don't be too worried, they are cheap to replace!
Its really not a hard job to do, just make sure you don't put any side load on the spool when you remove it, they are precision ground and straight to match the hole in the control block. If you bend it, its stuffed. Not that you would have to put any side load on it though, just don't try and twist it past any object that may be in the way is all.
Now, taking the spool to the dealer will do you nothing, the seals may run on the spool but they actually seat at the ends of the control block. The rear one may come out with the end cap on the spool but the other end will still be in the control block.