As I've been working on things, it dawned on me that pre-authoring is much like you describe, and that it's a tool that can be leveraged by in-the-moment authoring.
The way I'm looking at it now (and really, how I've been doing it in part, now that I've taken time to study it), is that in-the-moment authoring, or even what I might call 'reactive DMing' is dependent upon the skill of the DM combined with the resources they have available.
What resources? Well monsters, spells, magic items, etc. which are all provided in the core books in most systems (although you can always add/modify), campaign settings and supplements, etc. These are all more or less presumed.
But part of those resources is the prep work of the DM. Building a roster of interesting NPCs, with their own motivations and potential secrets. Ties to secret villainous organizations, and the motives and current operations of those organizations. What are they doing and why? Independent villains in the area, what are they doing and why? Maps for general types of dungeons - natural caves, towers, ruined keeps, crypts, etc. that are not assigned to anything in particular, until they are 'discovered' in the course of an adventure, at which point they now have a place on the map.
Basically, prepping can be an extremely helpful process for in-the-moment authoring, you have the NPCs, interesting locations, secrets and rumors, you just don't actually know where they are located. In the case of mobile things like NPCs and creatures, they could be in different places in different sessions. Physical locations, even 'known but forgotten ones' have at most a general location until the actual physical location is determined. If you want something to be in a specific physical location, that's fine too. But most of what's around it is still mutable.
Basically you pre-author a wide variety of building blocks to have available at a moment's notice as needed.
In addition, I think that the concept in the in-the-moment authoring that is often missed (particularly if we use the term 'fail forward') is that it's not reliant upon the failure of a skill check. New story elements, drawn from the resources prepared, or entirely improvised based on the situation, can occur at any time. It really has nothing to do with a skill check.
Having said that, some people advocate that a failed skill check and present one of these moments in time where a new story element can be introduced. It doesn't have to be, it can be. It's specifically allowing a skill check to be more than a skill check - it's a plot point. Whether that occurs because of the skill check, or a separate fate check, or the DM just adds it in is also irrelevant. It's recognizing that a skill check is a point in time where a result is determined - it's often already a logical point in the flow of the game for the players to discover something new, for an event to occur, or a setback.
However, by this definition, it doesn't have to be tied to failure, and as I said to an actual skill check either (yet another reason why 'fail forward' is not a great term). If we look at it from a broader perspective, it's looking at the exploration event from a wider angle and determining what more can be added that is more than just a simple check.
For example, during the course of a combat, there is great potential for a lot of things to occur besides just life and death. Capturing and interrogating an opponent, they can unwittingly divulge information, they can be carrying important documents, keys, or other objects, they could retreat and/escape leading the PCs to something important, etc.
Well, the same things apply to an exploration encounter. They can find documents, information, objects, or even creatures or NPCs depending on the circumstance. The point is that the DM is always reacting to what's going on, and that reaction can be the introduction of something new.
It's more about being prepared with enough material (or improv chops) to be able to react to the actions of the PCs. A well prepared DM will find it much easier to support this type of play.