MS Licencing is quite a big subject. I'm not sure about the AS/400, but to access MS Office, you basically need a licence for Office, a server CAL and a Terminal Server CAL for each device going to use the server for this purpose. So essentially you need a volume licence for Office, not OEM or retail.
This document explains: http://download.microsoft.com/download/1/7/7/17745e4a-5d31-4de4-a416-07c646336d94/desktop_application_with_windows_server_terminal_services.doc
Take note of the paragraph stating:
"Every device that uses Windows Server Terminal Services to remotely access Microsoft Office requires a Terminal Services CAL, in addition to Windows Server CAL and a Microsoft Office license. Dedicate a Microsoft Office license for every desktop on or from which you plan to use or access Microsoft Office, even if that use is only occasional..."
I think that providing you have the server CAL and TS CAL, then you'll be OK to access the AS400 from TS.