No news, sorry. Still working through my to-do list.
Not sure why I didn't think of this before, but if you want to write a viewer plugin using .Net then it's worth considering writing it as a Preview Handler.
Opus will already use any installed preview handlers. The API is very simple and there are no issues with writing them in .Net (they're already hosted "out of process" by Opus so there are no .Net runtime version conflicts to worry about, either).
As a bonus, your viewer would also work in Outlook 2007/2010 and (on Vista and above) Windows Explorer.
Some good info on Preview Handlers using .Net:
Sorry to necro an old thread, but I'd throw in my "DEMAND" as well 
I've written a PreviewHandler in .net and it works reasonably well with DOPUS except that I can't get it to kick in for any file type (since it has to be hosted in that "Activex preview office web" plugin).
Other than that though, it seems ok.
I had been hoping to rework it as a native DOPUS plug in at some point to resolve that limitation.
If it's registered properly, it should automatically be used for the extensions it's assigned to, in both Opus and File Explorer.
No reason .Net couldn't be used for a native plugin these days, as Microsoft have improved different .Net versions existing in the same process, and our process (and most others that display a File Open dialog or otherwise interact with the shell) is now full of .Net stuff due to various shell extensions. The main barrier would be translating the C/C++ API, which might be easiest done using C++.Net (either for the whole plugin or for a small layer that does the API bit).