Ok, so I found another way to do it, and I'll post it here for the ones who hit this question from Google in search for such solution.
What I did, is get the RDP ActiveX by id from the WebBrowser control, and set it's "Fullscreen" property to true:
HtmlElement rdpClient = myWebBrowser.Document.GetElementById("MsRdpClient");
rdpClient.SetAttribute("Fullscreen", "true");