I had problems with one of my axles and took the trailer in to a trailer co. to have the alignment checked. I asked them where to jack the trailer because I had the same question. They said you can jack on the tie plate (the piece of steel that ties the spring to the axle). It makes sense that this point would be safe because that's where the weight of the trailer is transferred from the springs to the wheel.
If you look at Al-KO's Owner's manual, page 30, you can see the layout of the springs and tie plate. Now if you have the configuration that has the tie plate on top of the springs, I'm not sure what to do.
This would also be a good question for Scott to weigh in on since the Al-KO owner's manual states you should follow the trailer manufacturer's recommendation. I haven't been able to find anything in the material I received that gives me that information.