3.1 comes with a rudimentary CDFilesystem. Get Icon Information on CD0 in the Storage/DOSDrivers directory and adjust the device and unit tooltypes accordingly. It should be enough to get a 3.9 emergency disk made. After booting from that I believe you'll be using CacheCDFS, which will allow you to finish the install.
EDIT: Oh, and remember to mount CD0 (doubleclick) after you've adjusted it. Drag it to Devs/DOSDrivers to make it permanent.
