I don't think the accepted answer here is entirely correct.
Right next to me is a barcode scanner connected to a Prolific USB2Serial adapter connected to a Windows 7 client (with installed Prolific Drivers). The Client sees this Prolific Adapter as COM3.
Now if I connect to any terminal server while allowing ports to be mapped (Options > Local Resources > More. Check "Ports"), that virtual COM3 will be available on the Terminal Server with no problem.
So to answer the question:
If you can install drivers on the Terminal Server Client machine and you install the Prolific driver, then you can enable Port redirection on the Client and the Port will be available and working correctly on the Server.
If you can't install drivers on the client, you will need an external 3rd party software as it was linked by @TheCompWiz