While I'm no "bones" expert, it looks what you are trying to do should work by doing a:

ent_animate(entity,frame,pct,0); // Set the frame to the pct.
ent_bonerotate(entity,"spine",vector(10,0,0)); // To twist (pan) the spine 10 deg

Although base on what I think you are talking about, you want to have a seperate animation from upper body (like aiming a gun), and one for lower body (like running), and blend the two.

If you make an upper body one where at 0 percent its aimming left, and 50% its middle, and 100% its right. And a lower body which cycles the run. You should be able do to something like this:

ent_animate(entity,"run",run_pct,ANM_CYCLE);
ent_animate(entity,"aim",aim_pct,ANM_ADD);

And that should do the same thing (without having to do the bone math yourself).