Just get a back up file that works with the current edition of the trainers.. You can get these files from "friends". Install the steam files once, then delete them and replace the files with the retail (patched) version of supreme commander.. Make sure steam updates for supcom is turned off.. Load game from steam and profit.. With update turned off steam couldn't tell if my files were actually from retail.
Store the file backups somewhere safe.
I can vouch that this method works, because I just did it 30 minutes ago.
[Edited by nick2412, 5/4/2012 12:59:58 AM]
hey friend, can you upload the .exe of a previous version that worked for you? so we can utilize the trainers posted by CH. thanks!