This highly, highly depends on the context of how the "Delay" works, as well as the incoming damage. The bigger the incoming hit, the better the "Delay" becomes as you will mitigate more and more immediately. The more incoming hits, the longer you have to deal with the damage as well as it gets continuously spread out into smaller hits instead of taking big instant ones. That 1 tick between hits can mean the difference between life and death if it gives you a small window to self heal or even just regenerate.
In this hypothetical, we can only assume that +MaxHP follows the same rules we see now where it has a finite limit to how high it can go while anything that directly negates incoming damage in some way can essentially infinitely scale upwards based on what's coming in. So in a vacuum, being able to split up damage over time will have a bit more value if given the choice. Though this would be incredibly reliant on how it works. Split 50/50 with 1 half now and the other in 5 seconds =/= Split 10 ways with 10% per second =/= take 50% right now, then 5 chunks of 10% over the next 10 seconds, etc. You are right that it may not give you immediate reaction benefit in a realistic sense, but over the course of a combat if any such rule applied to all attacks it would.
It'd definitely be an interesting tool to stack upon others, and I think this would be a far better tool than (funny enough) Dull Pain in Regeneration specifically. DP as is "works" in Regen for the reasons mentioned prior, but the set as a whole has a real hard time mitigating big hits, even with more HP. A damage delay mechanic in this set will probably be a huge help given how chunking out the damage gives its Regen time to actually kick in and actively fight the damage mid-combat. Granted, this would highly depend on how the damage is delayed, with smaller chunks over a longer period being ideal.
Assume you are going to take 4 bigs hits on a /Regen with 3 seconds between each hit. This is a total time frame of 9 seconds, so you get = (HP/S) * 9
[Ignoring time after the last hit taken as we can assume combat ends after that and you have infinite time to rest]
Dull Pain when slotted gives you a ~59% HP boost while active, which will boost you to = (HP/S * 1.59) * 9
For a Delay mechanic on /Regen to match this, we simply need to look at it this way = (HP/S) * (9 * 1.59)
Instead of boosting the Health returned per interval, you're increasing the intervals where you can return health. Given that the time frame of combat has not changed (9 sec), if you are able to deal with the situation within the combat window and take less damage within it, you're in a great spot.
As for the question of if Regen needs something like this instead of a DP-like power? That can be debated depending on how that would work. Something that say, chunks all damage you receive into 10% bites over 10 second durations would be much, much stronger than DP though that is a reach. Ideally, like with how +HP is essentially "+Resist vs all Damage", they would work together and not be an either-or choice in some form. Just because we get +HP in Regen doesn't mean you also don't get +15% res to all with Resilience, which is effectively +15% HP for 99.99% of the encounters in game. Stack those together and it's stronger than sum of their parts!
It does... and it doesn't. Like I mentioned above, 15% Resistance to all damage can effectively be seen as 15% more HP, and vice versa. However the bigger the incoming damage numbers, the more work +Resist does comparatively (though there are practical limits here for both and the only time this really comes into play is in a situation where you would have been pretty much one-shot).
Likewise, it doesn't make +Absorb not fit the power thematically either. Granted, I think the point of that has long sailed off as +HP in the current CoH meta has far more practical weight than +Absorb does so any Self +HP powers should stay +HP. Frostwork on the other hand may have a case to be +Absorb as it can stack upon other characters regardless of how much +HP they have and provide more potent mitigation.
I agree completely. Absorb has a place, as does +HP, and like with +Resist those places are stacked together to become stronger manipulatively 😄
In theory, they could have done a lot more with +Absorb if given time to flesh it out as a mechanic. If that were the case, I could see it being used more often or even replacing certain +HP powers depending on the whole toolkit and how it'd interact. As it is right now though, it is in a good spot as it stacks with everything else.