A few month back I had great trouble germinating some HBWR seeds. In the end I did the following to have one of the seeds germinate successfully: I took a pair of scissors to open one of the seeds at the bottom. I then put it in really wet soil, after this it took a few weeks to germinate successfully. The non-opened control seeds I had also put in the soil didn't seed even after 2 months so I recovered them from the soil to seed them next year, with the scissor tek of course.
Maybe this will work with your Iboga seeds as well. You can try it with one of them.