The accepted way to do it is with the 3rd party app, RemoteScan, from www.remote-scan.com
Works with all versions of Citrix and all versions of Windows Terminal Services.
From their web site:
With RemoteScan? software, scanners connected directly to client workstations can be accessed by any software running on Windows? Terminal Services or Citrix? servers. In addition, RemoteScan? is a software solution that turns any document scanner, ID or card scanner, flatbed scanner, or any other imaging device, into a network device right out of the box. With RemoteScan?, ordinary as well as high-end scanners can now be shared with other PCs or Thin Clients across your entire network.
Works with all TWAIN or WIA scanners and all software packages. RemoteScan installs in minutes, does not appreciably load network traffic and does not compromise network security.