The only data I know of close to this is that OSHA advises that workers are more likely to injure themselves when pulling vs. pushing.  A couple of thoughts would be;

1.  The relevant data would be the amount of force pushed, not the pounds on a pallet, cart or similar contrivance.

2.  Why is limited function of the APL or APB relevant for pulling?  Can a hook grasp be used, thus making the firing of thumb muscles not necessary? 

3.  Can the client pull the resistance with either hand?  Why limit what could critically be an essential function for a RTW case when the disability-not the impairment, is the critical question? 

4.  Could a one handed woman (you said "she") do this task?

5.  Does the course of action you are persuing exclude a qualified worker from a position in which they can safely perform (pulling)?

6.  Would the restriction(s) better read something like;

No forceful pinching with the left hand (greater than 3# of force) with the thumb opposed to fingers.

No forceful use of L hand (drills, jack hammers, elephan gun, etc) or work that includes forceful abduction or pinching with the L hand.  This form of information sharing describes the impairment (and ties the medical impairment to unilateral hand function for the reader) without restricting specific tasks.

Just some thoughts, hope it helps.