Ten years ago, two student from the Cornell University (North Australia) released the first public homebrew cart and software for the WonderSwan that does not use the WonderWitch. See here.
Their project was a major breakthrough as they where the first to document the WonderSwan hardware, and they are the original authors of the well known wstech document.
And after they made their release… nothing more. There were no concrete or public hardware development for the WonderSwan that went to a releasable product.