Sorry Casper, seems no one knows this one but I know you can freeze crab apples as Ive seen recommendations for this on a few forums but have never heard of anyone freezing a plum. Not that it cant be done just dont know if its good for that particular fruit.
Go ahead and freeze them. It's just going to make them mushy when they thaw. We've frozen apples for wine and it seemed to do them no harm. The worst thing that I can see happening is that it will break down the cell structure. We freeze choke cherries and that works out great!
I'm sure there are people here that know more about this then I do. But the recipe book I use recommends freezingall the fruit. It said that it breaks down the fiber and extracts more juice and flavor per pound. I don't know why it wouldn't work for plums also. I freeze everything that I use.
I have frozen plums before. I have a batch of plum going right now that had the plums frozen since last year. I haven't noticed anything different about the process. I always stock up on fruit in the summer, so I have no choice but to freeze it.
Do your prep work on the fruit before you freeze them though. Wash, trim and weigh them, so that when you take them out of the freezer all you need to do is open the bags and drop them into the bucket. We try to write the weight on the outside of each bag so we always knowhow muchisin them. Poor Bert and I are getting a bit old to retain that type of knowledge.
I wil take your advise and put the skins with the must.
I have so many wines to start when I come back from sea; Blackberry, Blackberry Port, Plum, Raspberry and some wine kits for blending. Can't wait.