I had almost the same problem and made it running today for a Win2003 Server, and now i must say it's pretty simple.
After installing the "RemoteDesktop over the Web"-Feature on the Windows Server you'll find a folder tsweb in your windows-folder. Copy the whole folder in your portal-Subfolder.
Now implement an iframe-module and "call" the default.htm file with the width=100% and height= .... it depends on the desired resolution you want to run the rdp-session.
For some reasons i modified the default.htm a little bit:
1.) i changed the standard port for rdp-ing in the registry of the server, you have to change this in the default.htm as well
To do so:
in the sub BtnConnect close to the end add the line
MsRdpClient.AdvancedSettings2.RDPPort = xx where xx is the desired port
2.) i changed the resolution drop down box a little, e.g. the default setting and the amount of possibilities at all.
To do so change the html-part of the file: (you'll find it very easy....)
<!-- Column 4 -->
<td valign="bottom"> <select size="1" name="comboResolution" id=comboRes class="topspace">
<option value="1"><id id=option1>Full-screen</id></option>
<option value="2"><id id=option2>640 by 480</id></option>
<option selected value="3"><id id=option3>800 by 600</id></option>
<option value="4"><id id=option4>1024 by 768</id></option>
<!--
<option value="5"><id id=option5>1280 by 1024</id></option>
<option value="6"><id id=option6>1600 by 1200</id></option>
-->
</select> </label>
</td>