No a dev kit is not a must, but when you start to port a lib as big as libavcodec and the only debugging mechanism is "printf" then debugging your code will take forever.
The status so far:
I have libavcodec "crosscompiled" with llvm-gcc and llvm to C89 compilant C files, that compiles just fine on the SDK together with some simple test code that loads a videofile and tries to decode the first 5 frames.
I have not even tried to run it cause it will probably only end in a black screen and I will spend forewer inserting log events to figure out where the problem is.
So I did the same for X86, crosscompiled to a C-file that VS2005 for windows understands.
So now I have libavcodec for windows and the same simple test program, but I get some access voilation when opening the file. This error should probably not be to hard to track down but I have not put any affort on it yet.
And then when it is running in windows the next step is to switch back to the XBOX.
The windows files is developed with a legal VS2005 copy, so I could distribute it freely if anyone is interested.