Hmmm, I found that if you null your speed and flip the right flags you don't take damage.
I tested this by launching myself on an absolutely suicidal trajectory (like 15 x and z difference) and pulling up the proj_list in gm-editor quickly at the top of the arc, with the right changes you just drop to the ground and only need to stand up, no impact or skidding or slamming into anything.
I tried to figure out how to trigger this at the very top of a jump in a couple ways, I got it where it doesn't error when I added these lines:
roof = proj.target_pos.z - proj.cur_pos.z
if roof==0 then
proj.speed_x=0
proj.speed_y=0
proj.speed_z=0
proj.flags.parabolic=false
proj.flags.has_hit_ground=true
unitSource.flags1.projectile=false
end
Because those are the flags/speed changes that caused me to drop from like 15 z up with no damage.
Unfortunately I'm not sure how to actually make it do a check like "if you're about to slam into something" or "if you're at the top of a jump" or "if your speed_z becomes negative" properly.
Note that I doubled the speed boosts because x/y/z*20000 let me consistently pass through the target_pos, though you do overshoot it sometimes, anyone got an idea how to make it flip those flags mid-jump when certain conditions are met, I was digging through the gui/gm-editor df lists to see if I could find a collisions index or something but I haven't had any luck yet.
Edit: nevermind, it isn't working now, not sure what happened but it's erroring me with those lines added in and I'm not even able to reproduce the "drop from way up in the air safely" result.