i've never heard of anyone leasing a claim, on tribute so to speak, around here.
i think the reason is in nz the obligations of the claim holder mean they are responsible for anyone working it i believe.
if you can prove your worth you might be able to provide testing for a claim holder, i.e.. running samples thru your sluice or pan at certain spots and passing on your finds to them. thing is most claims are for the river bed itself.