Congratulations. Well done! A good starting point.
I've been working on a DX11 driver for Spintz's 3Demon port of Irrlicht. I haven't had a whole lot of time to work on it but you can see an initial demo here: http://irrlicht.sourceforge.net/phpBB2/viewtopic.php?t=36732
One of the changes from DX9 to DX10/DX11 is colour swizzle. As you've found out some texture formats have been removed and channels are in a different order. Check out your vertex colours as well.
The DX Effect system has moved from an integral part of the SDK to being something you need to compile yourself, as you probably already know. I don't know about the current SDK but in the previous SDK I used I had to debug and fix the Effect code Microsoft shipped with the SDK as it wouldn't allow me to create shaders for DX10. I assume you've fixed that or it is not an issue.
I've also implemented a flexible buffer system for creation of buffers for vertices, indices, compute shaders and hardware instancing (stream support). My compute shader interface is a platform-independant interface from the Irrlicht device; it could support OpenCL from the GL driver for example, though matbe an OpenCL interface would allow for more efficient internals.
Flexible buffer formats is tricky - things like the MeshManipulator need to be rewritten to cope. The best way would probably be to give the manipulator a buffer and an output buffer declaration and have it work out what it can copy over from the input buffer and what it needs to generate (tangents, for example). With changes to Irrlicht internals to support streams you can do hardware instancing easier and more efficiently.
Anyway, when I get chance I'll look at porting my DX11 raytracer demo over as a test.