I'm having the same problem with an Epson GT-1500 and I've found a solution which works, although not the best solution.
I had to install special epson drivers for my scanner into my chroot area for the clients
Downloaded my drivers, copied them into my chroot section and installed with the following commands:
sudo apt-get install xsltproc
sudo dpkg -i iscan*
Once I had that installed I had to rebuild my ltsp image, reboot the client, then once the client was up I had to replug the scanner in. Then I could start simple scan locally and it would work.
Make sure you scanner is coming up on the client
Then within xterm
You should see your scanner in here.