the softlink works fine, as long as Apple does not try to change it in its update !
The way I found is the following:
cd /
sudo mv /Applications App_dummy
ln -s "/Volumes/Applications/Applications (MacOS 9)/Applications" Applications
Now my Applications folder is rerouted to the old system 9 applications folder. I still have to move the files that were in the OS X Applications flder to the old OS 9 Applications folder, and this can be done either with a "mv" instruction or in the finder.
It is safe and works for all users.
Now the problem is when you install an Apple update that will kill the link and replace it by a new folder (Apple, please accept to follow you user's links !). When it happens, copy the new files in the same location in the OS 9 directory (and do that for all files, including the ones hidden in the .app folders that are the new application files; you can open these in the Finder by ctrl-clicking on the OS X application files and choosing "open content" probably ("Afficher le contenu du Progiciel" in French )). When you are ready, move the /Applications folder in a safe place and rebuild the link (see above).