Here is an idea I am thinking about at the moment:
A current flows through the collector of Q2 into L2.
An increase in the voltage at the top of L2 causes Q1 to conduct more strongly, which lowers the base emitter voltage gap of Q2. The current through Q2 falls.
An increase in voltage causes a decrease in current which is negative resistance.
I need to experement with it more, in practice, but it seems useful for Regens and Oscillators.