In reply to Nick Smith - UKC:
a simple self-locking Z-rig can be made with the GRIGRI functioning as the first pulley in the system.
look at the diagram instructions provided and suspend from the reverso small loop as with magic plate locking off mode) and make the Z rig as describe above.
Not as good as the Gri Gri for the reason I have already mentioned BUT it will allow ascent. No easy descent.