TBH, I'm surprised we've never seen a proposal of the form "Federal employee salaries are not subject to income tax."
With a single line of legislation, that makes all federal job offers immediately 10-35% more competitive, without actually increasing salaries directly.
Once you get that as precedent, I could see it being used aggressively as a social tool: "We can't pay a public school teacher more than 60k, but we can pay them 60k tax-free, which is a little more competitive with a 75k private-sector position", for example.