(1) Happily, you can get the 4.1.0 file from the 10.5.8 Combo Update. (I don't know which of the previous updates introduced it — to save time, I looked only in this combo update, the last for Leopard.) Download the standalone installer from the Apple site.
(2) You must also have a file called "EPSON Stylus CX6600.ds", which belongs in "/Library/Image Capture/TWAIN Data Sources/". If it's not there, you can get it from Epson:
(a) Download the file for the CX6600 identified as "TWAIN Driver and EPSON Scan Utility v2.77A", with the filename "epson11802.dmg". It's described at the model's downloads page, and the direct link is "ftp://ftp.epson.com/drivers/epson11802.dmg".
(b) Epson seems to have messed up the content type of the file, or made some other error, because Safari at first tries to open it in the browser as though it's a movie. Give it a minute or two to sort itself out. If it never starts downloading, use an FTP program to get the file.
(c) Once you have it, mount it and run Epson Scan Installer.
(3) Next, go into "/Library/Image Capture/Devices/" and trash the file "Epson Scanner". (Well, you might want to just stash it somewhere else, in case you ever need to use an up-to-date Epson scanner.)
(4) Download a program called Pacifist, if you don't already have it. You will use it to extract from the 10.5.8 Combo Updater only the file that you need.
(5) Open the combo updater in Pacifist, and look for a file called "EPSON Scanner.app". Its location in the combo updater package is "/Library/Image Capture/Devices". Select this file and click the "Install" button in the toolbar.
(6) "EPSON Scanner.app" should now be in your Devices folder. Double-click on it to launch it. (It runs in the background, so you won't see an icon in the dock, but you .)
(7) At this point, you should be able to power up and connect your CX6600, launch Image Capture, and scan as usual. (The EPSON Scan application, which will have been installed in your Applications folder during step 2, if it was not already there, should also function.)