Hi,
actually, it's all there for you. In the IK example on PlayMaker samples, instead of manually turn on/off ik, you turn it on and off as the user gets close to a wall.
also, you only trigger IK for the arm in question, not for all limbs.
Approach this using small steps.
1: can you modify the IK example to only IK one arm, the rest would remain totally fine and under mecanim animation control
2: can you switch from one arm to the other. that is, you have two events, one to toggle IK on the right arm, and one to toggle IK on the left arm
3: can you trigger that event based on raycasting ( us a simple cube that you move around the character so that raycast finds it
4: can you moe the IK end effector on the raycast Hit position
if you can do all this, then you are set
each of these steps are really manageabl. Howevr don't hesitate to ask question on these particular steps as you go, I'll assist.
Bye,
Jean