Nop.
You can not get the relative bone orientation with that.
It seems to me that the only way to get the relative bone orientation atm is to set the entities pan/tilt/roll to 0 and than use ang_for_bone and then set the entities orientation back.

But this causes somehow the entity to be rendered with an orientation of 0,0,0 instead of its true orientation, which seems to be bug.

And since there does not seem to be an accurate way to calculate the relative bone orientation from ang_for_bone and the entities orientation
[ the result from ang_for_bone is ang_add(relative_bone_orient,ent.pan) ]
--> the problem still exists.

Sample project