There is no motivation on the defense contractor side to do anything more than satisfy the requirements of the contract. And any R&D spent should result in an interesting demonstration that brings in more business.
Standard operating procedure would need to change so the government entity has security as a requirement, details on how the requirement can be satisfied, and a bunch of money to pay for it.
So tack on $X million for each contract to have a 3rd party audit the code, documentation, and hardware for security vulnerabilities. And an added maintenance contract to fix any future vulnerabilities for the lifetime of the program (20+ years most likely).
From the higher up side, what do you get for all that money spent? No new functionality, no fancy demos. Going to be hard to convince them security is important when they can fund something they view as more critical or more interesting.
EDIT: To answer the question of what can be done, I think it'd require a culture change on the contracting side. The engineering side of the house is mandated to only do work that relates directly to the contract. The hours bid will likely be for the minimum necessary to satisfy those requirements. You can create a new interface, but you won't have the time to do any fuzz testing for example.