I can find several papers in IEEE Aerospace Conference proceedings, but they are not available in free form online. This is a shame, because the government owns the copyright.

Unless something has changed, the US government doesn't own the copyright - works they produce are automatically in the public domain. Most journal put wording on the paper to that effect.

So if you have access to the papers, you should be able to post them for free somewhere without violating copyright.

Not quite. They were almost certainly approved by JPL in its role as a FFRDC. The copyright in such cases belongs with the government. What you write would be correct if they went through copyright release at another NASA center like JSC in which the workers are federal employees (I work at JPL.)