Income tax: capital gains: can property or a right, the 'market value' of which is nil, be a CGT asset?

Yes. Property or a right can be a CGT asset even if its market value is nil.

The existence of a CGT asset is not dependent on its having a market value greater than nil. The definition of 'CGT asset' in subsection 108-5(1) of the Income Tax Assessment Act 1997 does not impose any such condition.