Hi,
I haven't done this myself but Pos .Net supports a feature called 'Device Redirection' that is specifically for this situation. I understand it works with Microsoft Terminal Server, I do not know if it works with Citrix but would hope that it does.
The Pos .Net SDK should contain help on the matter (I haven't got it handy now to check), but the online help does mention how to set it up here;
http://technet.microsoft.com/en-us/library/cc732188(WS.10).aspx