The solution we found was to replace the Universal version of EPSON Scanner.app on the iMac (version 3.1.1) with the PowerPC version (version 3.1.0) from a G4 mac running OS X 10.4.11. We then modified the plist files as described in the original hint, and were able to scan with the Stylus 2500 using Image Capture.
[robg adds: Note that this change will have the scanner running in Rosetta mode on the Intel-powered machine. As with any change to a system file, I would recommend keeping a backup of the original, and you're on your own if this causes problems with your machine.]

