my first idea to this is that you scale the ANGLE
ANGLE target;
var percent =0;
ANGLE temp;
while(1)
{
percent += time_step;
if(percent<1)
{
vec_set(temp,target
vec_scale(temp,percent);
ent_bonereset(me,"theBone");
ent_bonerotate(me,"theBone",temp);
}
else
{
ent_bonereset(me,"theBone");
ent_bonerotate(me,"theBone",target);
break;
}
wait(1);
}

something like this
muffel