It could be done, it's just probably not worth it. You can target Python code to .NET using
IronPython, but the entire UI would have to be rewritten, most of the storage subsystem would probably have to be rewritten, components like the content server don't really make sense on RT, etc.
Windows RT is roughly equivalent to Android or iOS, more focused on consumption rather than creation (where calibre falls under the "creation" category, since you're creating and managing libraries, doing conversions, serving content, etc). Calibre on Android and iOS doesn't really make sense, nor does calibre on Windows RT.
That said, there are ebook readers like Freda available for Windows RT that can read from OPDS catalogs, and you also have the option of opening files from your local network via SMB (something neither iOS nor Android have built-in, but that Just Works(tm) in Windows RT). Run calibre somewhere else on your network, and consume the results from Freda.