Snow!

If you had a fly, a snow bag would be your best option. (this is for future searches) A snow bag is a long piece of muslin around 40' long by 10' with grommets and ties on the top and bottom. On about 1' centers you cut slits that are about 2' long centered on one half of the muslin. You tie one side of the musling to one batten and one side to another batten forming a U. You fill it then fly the side with the slits on it out so the snow gathers on the side that does not have the slits. You fly it out. When you want to operate it you fly the "working side" (side with slits) in so the snow now gathers near the slits. You then slowly move the batten in and out to make the snow fall. This is usually the traditional nutcracker way of making snow on stage.

Now... for you.... get some plastic netting of this variety. http://www.internetmesh.net/plastic_oriented.htm#search

Run a rope across the stage on the ceiling. Form a U with the netting with the snow inside. Run the rope through the holes of the netting so that the netting is held together and up on the rope. Shake the rope by pulling it tight and loosening it should cause the snow to work its way out. This method works, but you will get the occasional clump that comes out along with random snow falling occasionally.

 

It all depends on the size of the snow. You want to get netting that is slightly smaller then your average snowflake so when you are not shaking it nothing comes out, but when you shake it it will agitate and fold the snow and it will fall. Its not the most perfect system every, but it does work.