The hardest part of making the Fujitsu ScanSnap S300 scanner work in Linux with SANE was to find the firmware file that is required to make the scanner function. Without it, scanimage -L won't even try to recognise the scanner. The configuration file /etc/sane.d/epjitsu.conf has to have a "firmware" stanza pointing to the right file.
There are pointers for finding the file in the aforementioned configuration file, but at least for me, things were a little bit different. After installing the driver on a Windows box, the firmware was in c:\WINDOWS\SSDriver\S300 with the name 300_0C00.nal (not 300_0A00.nal as advertised in SANE's documentation).
As not every ScanSnap S300 owner has a Windows box available, I put the firmware on my web page for download: http://members.sange.fi/~atehwa/300_0C00.nal
(I don't think there's anything evil doing so, because the firmware is of little use without the physical device.)