For someone to come up with this idea several things are needed:
Knowledge that demon summoning exists. This is an easy requirement.
You also need knowledge of how time interacts with it. You need to know that the demon takes action 5 days before the spell is cast. This is not that likely to be common knowledge in the realm, since there aren't that many dedicated demonolgists (rather than dabblers).
You need to know the existence of a obscure necromancy spell, White Bone Emissary, and that it provides instant communication. The person who created this spell is Ma-Ha-Suchi, and we don't know if he taught anyone else the spell. He definitely isn't about to teach it to the anyone from the Realm.
Now someone knowing all these things, doesn't mean that they would come up with the idea. That an idea is simple to understand, doesn't mean that it will easily be discovered.
There are other difficulties in pulling it off.
To make it work you also need a Demon that won't randomly be summoned by someone else, and that you know the identity of. The demon may be summoned by something else. The chance of this happening is less if it is a higher circle, but Dragonblooded can't summon those. If the Demon is a more obscure type then it is less likely to be summoned by someone else, but that means it's also less likely for the sorcerer to know about it.
The demon in question has to be staying in one place so that the necromancer can notice that it is being summoned, and you may need to tell it what you are trying to do.
First, you've underestimated the value of the hundreds to thousands of years of accumulated research on demons that the Realm has. Seriously, if decent portions of any sorcerer's libraries survived the Shogunate, you're looking at potentially thousands of years of knowledge. Frankly, this would probably be something that first happened in the First Age (which also brings up the problem of this not being Usurpation-OK, but whatever).
(Really, unless whatever exploit you're thinking of absolutely requires something from after the Usurpation happened, you should assume that the First Age abused the fuck out of it.)
Second, you've underestimated the value of foreknowledge for any organization, especially a nation. If the fact that demons act before they're summoned is known, lots of effort (which can reasonably be a personal project for
multiple centuries) is likely to go into figuring out how to exploit that fact.
Third, apparently Ma-Ha-Suchi is the only necromancer who went "gosh, I sure wish I could cast Infallible Messenger"? Nah, dude. Also, propagation is very much a thing, regardless of the desires of the origin.
Fourth, you don't need an obscure demon; a useless summon works just a well, and an obscure
and useless summon provides the benefits of both; further, as mentioned, I'm pretty sure there
are ways to prevent other people from summoning your demons.
Fifth, demons are super bribable. No need to put lots of effort into talking them into it when you can just bribe them into doing what you want.