Ok, thanks ratters, maybe I will just have to go get enough from that city park this summer to make a batch and see how it goes.
You can also try your state extension for info. They might even be able to ID the park trees. I found this from a quick search: Short Seaon Idaho Growing.
Try winemakingtalk.com and winepress.us for more wine info. Try gardenweb.com forums for Plum growing info.