Yeah, I've done it for the whole domain too. You just have to make all the mail accounts regular users and give them .procmailrc & .postfix.forward files. They can just use the path to where ever you installed SA.
If you want to easily train SA on false negatives, make a "LearnAsSpam" folder as a direct child of your inbox, and put this in your crontab (all one line):
X Y * * * for file in
ls ~/Maildir/.LearnAsSpam/cur; do ~/bin/sa-learn --spam ~/Maildir/.LearnAsSpam/cur/$file >/dev/null && rm -f ~/Maildir/.LearnAsSpam/cur/$file; done
This assumes sa-learn is in ~/bin/ and you should change X to some number between 0 and 59, and Y to some number between 1 and 6.
Training on false positives is probably not that important in the whole scheme of things.