Have we gotten good enough at jet engines to be able to generate enough thrust in the upper atmosphere to turn planes into ICBMs for an ex-atmospheric inertial jaunt?
With ramjets/scramjets maybe, but developing the inverse of what you suggest is where the money is being spent. Missiles on ballistic trajectories are easier to see on radar and have less ability to maneuver, so they're easier to intercept. So instead of starting out breathing air and then pulling into a ballistic climb at the end, the idea is to start out with a rocket powered ballistic flight, reenter the atmosphere, and then glide/fly through the atmosphere, possibly with a ram/scramjet, to the target. This way the missile is much harder to detect and intercept.
Like this image depicts: https://en.wikipedia.org/wiki/Qian_Xuesen#/media/File:Tsien_... (This sort of missile flight is sometimes called a Qian Xuesen trajectory, after the man who first came up with the idea.)