The Wii came out 5 years ago, but the machine itself isn't much more than a reboxed Gamecube, so it's essentially 10 year old hardware by now.

Yes, the GPU has been used to accelerate video encoding. In fact I worked on such a project myself in 2003! But there was a GPU accelerated video encoding patent already back then - although one that works differently. There are in fact some tricky implementation details in this Microsoft patent - ones that might not qualify as trivial. But those non-trivial parts rely on DX9 functionality, using vertex and pixel shaders to solve some problems in a tricky way. An OpenCL or CUDA implementation wouldn't need to use any of these, so the whole patent is irrelevant today.