Shock absorbing lanyards are not suitable for a fall restraint system.

The problem I have with that is the caveats around the likely forces imposed during the work, The WAHSA guidance says the same, you have to ensure that the forces involved cannot make the shock absorbing bit deploy.

If you use the correct harness then you do not have to worry about that aspect.

I have investigated an accident where the absorber deployed sending a worker through an atrium roof so it does happen.

You would need to check with the lanyard manufacturer.

It might vary however, the ones [shock absorbing lanyard] I have use in the past will not deploy at forces below 200kg and have a weight limit up to 140kg.

To give you an idea of the forces required (I am aware that restraint does not allow a person to fall this is just an example as shock absorbing lanyards are mentioned to help understand what they are desinged to do) they are designed to keep the force limited to 6kN

a 140kg mass falling 1m will exert 1372N into the system which is 139.91kg

a 140kg mass falling 2m will exert 2744N into the system which is 279.81kg 

It will also depend on how the restraint system is set up.

have you looked retracable lanyards?

TommyH the answer is no as the inertia will not work.  The manufacturers paperwork that came with the certificate of conformity states it should not be used vertically.