But this is irrelevant to
@Neonchameleon's point. You can have secret DCs in a 3E-style resolution system. Unless you also think it's really important to keep the bonus secret too - but why would it be?
The players know their own bonuses but they don't know of any hidden modifiers (when such are present, which isn't always) and I'd like to keep that info secret for as long as it makes sense to do so.
I really don't know what sort of "kitbashing" you have in mind. But even on its own terms AD&D is highly vulnerable to breakage - eg the GM who allows ability score checks to overlap in function with thief skills; or the lack of fit between surprise resolution for monks and the rules for determining segments of surprise; or the risk of broken magic items, which Gygax repeatedly warns against in his DMG.
1e is IME amazingly resilient to kitbashing, even when such kitbashing isn't done well (believe me, I've loads of experience with badly-done kitbashing having been the author of great gobs of it over the years!). But obviously, anything can be broken if one tries.
One point of 4e's resolution framework is that it makes many separate subsy8stems redundant. There's no need for different systems for (say) finding and disarming traps; evading pursuit in the wilderness; and befriending a NPC - these can all be resolved as skill challenges.
They can if one wants, though of those three the only place I'd use a 4e-style skill challenge is the evasion of pursuit, for which I've yet to see anything better.
But if for whatever reason you want to create a separate sub-system for reaction rolls in 4e, that's not going to be any more work than it would be in AD&D. It might be desirable to consider how to factor Diplomacy skill into it; but then, in AD&D you'll need to work out how to factor CHA into such a system, so I don't see any marked difference. Or suppose you wanted resurrection survival checks, you could just copy the chart straight over from AD&D.
I guess I'm really not clear on what these "knock-on" effects are.
Some changes like those you mention can be easily added in (and IMO the d% res. survival roll should be!). Others can't.
Using an example from the 3e game I was in: the DM wanted to make the campaign last longer and so he slowed down level advancement immensely, to similar to what our 1e-style games had.
Knock-on effect: wealth-by-level went out the window.
---Knock-on effect: eventually, most other wealth measures e.g. wealth-by-town also blew up.
---Knock-on effect: attempts to fix this on the fly by reducing available treasure quickly led to huge wealth imbalance within the party, newer characters (we had a slow but steady turnover both of players and characters) had nowhere near what the veterans had and no chance of catching up.
---Knock-on effect: by mid-level, CR and EL guidelines also went out the window due to how much wealth and gear we had.
Knock-on effect: some adventures became too easy for us at the start and-or too hard at the end; the authors expected the party to level up a few times during the module, which of course we didn't. This caused some DM headaches and probably a few PC deaths before he realized he had to start tweaking encounters.
In 1e, where there's no such thing as wealth-by-level guidelines and thus nothing else is predicated on them, changing the advancement rate wouldn't have had nearly the same consequences elsewhere.