Hi there jr_really,
I don't really have any answers for you but i know how you feel. Sacroilliac joints seem to be very hard to treat.
As for cycling- have you tried a recumberant bike? If not for riding on the road then just for training (e.g. at a gym if you go to one)? At least you might be able to get some good exercise that way.
In my personal experience i have found walking helpful for correcting SI problems however this may not relate to your situation. I'm sure you can tell what aggravates your problem.
Only suggestion i have at the moment is try to find someone who is doing research like at a uni or something. If someone is doing a study on low back pain they probably would be grateful for volunteer participants and they are probably challenging common treatments and beliefs. Perhaps you might find a professor or someone in a uni who has done research already and is a specialist in this area and can help you