I just sort by the distance of the front bottom edge of each object towards the origin of the scene.
Slopes don't get sorted right and I'm unsure they ever will. Maybe I should just ditch them. Slope movement is achieved by changing the evaluated height (w) of a platform by the distance from the platform hotspot (dx) multiplicated by a slope factor (dwdx), so w=dx * dwdx
Since I'm rather happy with the sorting now (not perfect but works for certain object sizes), I added some other stuff. Jump is now uncontrollable on air, I guess this should be a toggle. There's a moving platform and the bridge pieces accelerate upwards while you're standing on them.
All of this is to show what can be done here =)
Same file location as always. http://octavoarte.cl/25d_concept.exe
Make sure to refresh your browser cache if you seem to be getting an old file.