Huge bump on this thread. But while reading it I had an idea which people never tried for removing the IHS. Why not using some chemical, that reacts and dissolve solder, but not silicon? So all that it would be needed would be drilling a little hole on the IHS, or inject it with a needle below the IHS(removing the glue first).
I think there are plenty of acids able to do that. Silicon is unaffected by most acids, including sulfuric acid.
That would be definitely less dangerous to the CPU than desoldering the IHS if the PCB is also acid resistant. If it's not I think it would still be feasible be slowly removing the IHS area over the die and adding acid until all the solder from the die is removed, without dropping any acid on the PCB.