I take this to mean that the air/raft's propulsion is somehow inextricably linked to felt gravity. Propulsion efficiency drops in close proportion to the felt gravity. Once the latter reaches microgravity strength, the former essentially ceases to function

Felt gravity is everywhere. Where do you think gravity drops off to "microgravity" strength? I presume micro means 10^-6 of a g? If that's the case you're looking at 1000 earth radii for Earth.