I'm writing up setting details for a Demon the Descent game, and I have a couple pieces of Infrastructure that, currently, have people as their Linchpins. Cultists embedded in the mortal functions to use the pieces for the god-machine (a recruiter for the cult's experiments, and a bureaucrat covering up sacrifices as accidents). Can people be Linchpins, or does it have to be an object?
Most of this comes down to the basic information on Infrastructure.
DtD page 66: "Infrastructure doesn't operate under the laws of physics as ordinary mortals understand them, though."
DtD page 63: "The God-Machine piggybacks on mortal infrastructure when it can."
However, Flowers of Hell adds a couple twists:
On page 67, there are multiple examples of Infrastructure/Linchpins. The first example calls out a captured Demon as the Linchpin. The final example says "[Which police officer is] pulling the strings, and acting as a Linchpin?"
Based on this, here's what it comes down to:
When you have Aether-producing, techgnostic, freaky capital I Infrastructure, the corresponding Linchpin is going to be, naturally, unnatural in some fashion. It would appear as though living things, at least, can be found as Linchpins in this sense.
However, when it can, the GM will use normal, mortal lower i infrastructure, to deliver what it needs when it can. Naturally, ordinary people can be the linchpins of these setups.
In summary: for a step in the GM's designs - a discrete link in the chain - if it's a mundane construction, a normal human can certainly be the fulcrum point. As you get into occult matrix producing Infrastructure with proper Linchpins, the pivotal element becomes something distinctly non-natural. And I think that's the most conclusive we can get right now.
Also, for your specific example, in FOH, page 70, the second example is a cult recruiting endeavor - the Linchpin is the testing room (the tests are activating some latent psychics.)
Let's talk about Infrastructure.
Infrastructure serves the purpose of taking an input and deriving an output. Each piece of Infrastructure has an "eye of the storm" Linchpin that is crucial to the output. But every piece of Infrastructure is following the gnostic laws that govern reality. People can't comprehend why those laws work, Demons can sometimes make more sense of them, but only the GM knows in full detail. However, most of these laws generate Aether as a by-product.
For example, if you hit a steak cut from a cow that has not been exposed to sunlight with a specific meat hammer, it will shatter into 20 lb. of diamonds. That's a scientific fact.
To build that into an Infrastructure, I'd hide that in a meat processing plant, selecting one worker to give the special meat hammer to, and describing how to not be surprised by the diamonds, and to instead collect them. (Those diamonds are the input for a whole different Infrastructure)
So, what is the weakest link here? Sure, if the moonlight cows (supplied by another piece of Infrastructure) were to dry up, my diamond factory would likewise suffer. Sure, you could kill the worker, but that just leaves a job opening. Stealing or destroying the meat hammer would completely stop the process, though, so it sounds like the meat hammer is the Linchpin, in this case.
So, if there is a Concealment Infrastructure that is covering up human sacrifices, while a human is stamping the documentation, perhaps a better linchpin would be the stamp he uses on the official paperwork. Embedded inside is an eye sewn shut, and, when the stamp is applied to a personal file, documentation shifts to include reports of previous workplace injury or the like, throwing off casual, mortal investigation.
Now, I said the moonlight cows came from Infrastructure, but that doesn't have to be true. Cows of this quality could be generated without "supernatural intervention." And that's fine - without the pesky Aether run off, those former servants of mine as less likely to come trying to huff off the power of my moonlight cattle ranch.
Likewise, a human recruiter recruiting humans doesn't need to be a piece of Infrastructure. However, give a man a clipboard (that has five CPUs wired in a pentagram formation on the back, covered by a sheet of paper) that causes whoever signs it to have euphoric dreams, pushing them into joining a cult, that most definitely could be a Linchpin for a piece of Logical Infrastructure.
Now, maybe this clipboard only works when it is offered by a man who was born during an eclipse while six people chanted around his mother. Maybe the stamp only works for a person who has seen three significant others killed in front of his eyes. Maybe people can be so important to the Infrastructure that they essentially become Linchpins.
First off, that's likely to require them to become Stigmatic. This may or may not be desirable.
From the GM's "perspective", here are the cons to human Linchpins (or Infrastructure that revolves around specific humans to the point that they are essentially Linchpins)
- The more specific the kind of person, the harder they are to replace.
- Humans are super squishy, and replacements could take time to find/create.
- Assigning Guardians to them may extend their lifetimes, but could end up with a rogue Demon who then knows far too much about the Infrastructure's weak point.
- Even assuming they somehow survive for a long time, they will still die.
However, there is a plus side:
- Demons don't really expect humans to be Linchpins (because of all the inherent drawbacks thereof)
But again, one of the key themes to the Demon game is techgnostic mystery. Perhaps your troupe discovers what they think is the Linchpin, and try to suborn it, only to discover that some unknown property of the Infrastructure has bonded it to its primary operator. This surely is some freak occurrence even the GM wasn't anticipating... or was it?
Also, since you only mentioned the core book, I will point out that Flowers of Hell (the Demon player's guide) has a lot of material on Infrastructure, and is worth a read.