Does the middle way apply to objects as well as the skandhas? Does the middle way -- between annihilationism and eternalism -- apply to objects? Like a mountain, will it either exist forever or be destroyed, or is it too the middle way?


  The expression “middle way” refers to the Buddhist understanding of
  practical life, avoiding the extremes of self-denial and
  self-indulgence, as well as the view of reality that avoids the
  extreme positions of eternalism and annihilationism.


That is one way of reading the meaning of 'middle way'.

Do objects -- supposing they exist -- we have consciousness of have substance, according to any Mahayana Buddhists? That seems to ask the same question, I'm not sure.
The middle way -- between annihilationism and eternalism -- does apply to objects. Objects are neither existent nor not existent. They are interpretative imputations made on the basis of multiple causes and conditions temporarily coming together in their non-stop motion, transformation, and interaction.

Objects we have consciousness of are not "made of" substance, according to Mahayana Buddhists. Objects are interpretative imputations made by the observer. The so-called material substance is but a small part of all conditions, combination of which leads to arising of this particular delineation and designation.