George had a thought in comments on the last post that could easily be relevant. If SpaceX starts reusing the Falcon9 cores, which are the cheapest cores in current production, then how much more financial sense would it make to reuse the SLS cores? The success of SpaceX could put pressure on NASA via the taxpayer to save the hundreds of millions of dollars per launch of the SLS system.
NASA may be the wrong target. ULA has quite expensive Atlas and Delta cores that they should have a financial interest in recovering. The Atlas could use some fairly small kerosene/LOX engines from in-house or any number of suppliers, including XCOR and Masten. Any number of small firms now have in-house expertise on vertical landing systems. Adding a small number of pressure fed engines for the landing sequence would add weight and complexity, with these engines optimized for sea level operation could also be used to increase allowable GLOW.
After main engine cutoff of the booster, residual propellant could be pumped into one of the empty helium spheres. There will be time in between main engine cutoff to compress helium from the main tanks into the new repurposed landing tanks. It seems possible that a minimal amount of hardware would but need to be added to the stage.
The Delta system could use the RL 10 and Delta Clipper software so as to use the same hydrogen propellant as the RS 68. This could use known systems to recover quite expensive hardware.