User blog comment:Sinthorion/Vanilla Item / Block obfuscated code name translation/@comment-25612015-20150916193820

You have the full list of vanilla Minecraft fields in Mod Coder Pack 9.08, in file conf/fields.csv.

I think if you feed the Minecraft source in MCP to your jar decompiler, you should be able to load all the missing classes with their meaningful symbols.