Besides that, use open protocols and open source for the whole stack. Use open source hardware too, but don't depend on being able to order new hardware. Processes could change (we could move away from silicon to something else), or file formats for schematics can change so that you can't find anyone to produce the product.
Also helpful would be to target a common VM like the JVM instead of relying on hardware-specific abilities.
I've never read through those contracts myself but I would hope that they're public record if they're so old and important.
Interestingly, there's case studies in the private sector for doing exactly this sort of thing in cyclical industries. Toyota is famous (within the rarefied air of supply-chain textbooks, anyway) for supposedly buying up its suppliers parts and stockpiling them, or even just paying the suppliers to be idle, so that they don't go out of business (or start retooling for other clients) during lean times and are ready to go when demand returns. If you view the military-industrial complex as basically a "supplier" to the US Government, and view the demand for military equipment somewhat cynically as a cyclical demand curve, the government's behavior makes sense. You don't let your suppliers go out of business if you think you might need them again...
That's really the best way to do it; you have to own (or control) as much of the supply chain as you can. Not really practical if you're a private company who has to compete on thin margins, but not really that hard to imagine for a government.
As far as impracticality arguments: keep in mind that each government that maintains a nuclear arsenal (rogue/client states excepted) already has a completely captive technological supply chain, generally at least at a 1950s level, in order to manufacture the weapons in the first place. So if you wanted to build secure nuclear C2 systems, it would make sense to basically build them with the same degree of security, and using the same presumptively-secure supply chains, that are used to create the weapons themselves.