I found myself asking the same thing as @ptd in recent months. Finally I found an Adobe After Effects script that converts an AE ease to Greensock custom ease, such as:
M0,86.4187C32.9957,86.4187,0,-1297.5813,91,-1297.5813C93,-1297.5813,96,-1297.5813,97,-1297.5813
https://github.com/SupportClass/ae-ease-to-gsap-customease
Hopefully this helps developers who work in tandem with interactive designers, where there's a handoff of interactive requirements/mockups.