tl;dr - It probably would require a substantial rewrite of combat because it turns combat into a long, drawn out grind. That being said, I doubt they would put it in the DMG because it would require "too much work."
My personal take is that the current system of attack roll vs. AC tells where armor/Dex avoids the hit entirely. Damage dice tells how deep the cut/potent the spell was.
I personally don't think so. Combat as it is was written with a curve for Armor Class in mind, not with damage reduction. It would likely require an entirely different combat system, or at least, a damage progression system for characters. Besides, it doesn't make any sense for how hit points are actually described as more of a fatigue factor than a damage one.
I don't doubt they will have it, but I agree with you that I don't think DR is as easy as armor reduces X amount of damage system.
I played many games that had DR systems and it wasn't fun (or it was easy to become invulnerable)
you need to reduce the large amount of Hitpoints to start with. then make sure one armor type doesnt make you invulnerable to attacks. It isnt a simple system.