You've got a nice game concept there
1) Do a c_trace instruction below the player (the usual way) but don't set it z according to the result; simply decreases with (let's say) -2 * time_step each frame if there's a distance between its feet and the ground.
2) Use c_trace to detect the texture below player's feet. If its name is "my_rotating_thing" (or so) move it together with the rotating thing. I've got a "walk over moving train" snippet in one of the recent magazines' faq - it might help you.