A .NET WPF player takes about 500ms more to load so for instance is available after a second instead of a half second. It takes more memory but but less disk space. You can try my private player StaxPlayer if you have doubts in efficiency. The main dialog uses WinForms because of WinAPI interoperability, dialogs use WinForms which does standard controls well and the OSD uses WPF which does fancy design well. The WPF OSD is done with separate windows that sit on top of the main/video window so it's independent from renderers. DirectShow is a COM API so it can be easily used from different languages. Technically a WPF OSD for MPC would be no problem, some just would need to expose a COM SDK like DVBViewer or MediaMonkey. That's one way, GDI and DirectX would be another, even WebKit might be a option, Google builds a HTPC interface with that, it's called Google TV not released yet.
http://www.filedropper.com/staxplayer