I have a friend (really!), who is like 190 cm tall and because of hard work he has damaged his spine on top of his scoliosis.
When he asked me how to train abs the only thing I could come up with are hanging leg raises, this little roller that trains the abs without putting load on the spine and this crunch thing that actually forces you to train your abs if you want to or not. I try to find a picture of at least one.
However, since you know things, what would you tell someone who cannot put a heavy weight on his spine to strengthen the posterior chain and his abs?