I believe you could setup the IK constraints, animate, then go through and key the rotations for the IK bones, then remove the IK constraints. This is of course a pain if you need to modify the animations later.
Otherwise there is no way to automatically bake the IK constraints. Is there a reason you are trying to avoid IK to runtime? I very much doubt if the IK computations are a bottleneck. The IK constraint code may be long but the most common code paths taken are short.
