Is there a term for the general method used in XORed linked list?
It is a typical example of a space-time tradeoff trading increased time (CPU instructions required) for decreased space (memory required). There are many examples of this.

In this particular case I'd say it is a (simple) form of lossless compression. This because the data is stored in a minified but unusable form and needs some computation (decompression) done to receive the original (and usable) form, but no information is lost.