Okay, so now I think I got the basics down. First I would set up a simple sliding mechanic for the inrun, where the actor just glides and gradually accelerates down the slope. This should be accurate, as I have heard athletes describe the inrun as "virtually frictionless". For the takeoff, I would have a very thin "launch pad", and change the speed and height of the jump based on how accurate the player times the jump, something I don't really care to explain right now. Then I would adjust the speed and lift with a linear falloff based on the angle of the ski jumper. As of the moment I'm not sure how to measure my angle, maybe I could use traces?
The landing part should be more related to animation, something which is not too much of my problem now and should be easy. I'll just do a on "click left do telemark" and on say "click both do normal landing". Should be easy. I'm gonna do some testing soon, stay tuned for updates.
TL;DR I think I’ve got a solution