In another thread here someone suggested to have implemented a mod which is able to „teleport unwanted multiplayer aircraft to flight level 250“:
https://forums.flightsimulator.com/t/top-gun-escort/521724/15
So is it actually possible (now) to get access to the list of multiplayer aircraft via SimConnect (with the same or different API that reports AI objects)?
Hint: Nobody was mentioning Simconnect before you brought it up … 
A pity. But that essentially answers my question then already
(I am only interested in SimConnect for now).
On a more positive note, there has been a LONG time request/wish, almost since the day MSFS was released (or even before the release), to allow access to the MP Traffic data, and the Traffic’s parameters, but despite being requested in Forum Wish Lists, and asked repeatedly in Q&A, it has never happened.
So, for example, it is not possible to ADD Tags to Vatsim Injected planes, or (easily) modify what tag information is displayed in MSFS MP planes.
I don’t believe there is an API to allow access to the Connected MP Plane list, but obviously since MSFS can display that information in its UI, that data is there for each MP object… but not accessible by any documented Simconnect API.
BTW, That UI needs a little TLC – but that would be way down on any priority list