Dear Johannes,
I really wish you were around!
We at segaXtreme are working on a lot of things that would be very helpful for an eventual SGL replacement library.
Not only is there libyaul (an SDK), I have also written a functional sound driver, and XL2 has helped me build a render path. He also made a model converter which I have forked.
I'm just letting you know that because of the springboard Jo Engine was to us, we've gone forward and made progress with Saturn libraries, but we're all in our own park.
As soon as you have free time, we would all love to help you integrate these things back into Jo Engine and make it not only easy, but truly great.