Exactly, looking into that already, but it seems SimConnect can not do that for every scenery object, only SimObjects can be placed with SimConnect while FsX is running. But I'll continue my search

.
That's right, but you can create "dummy" simobjects containers for each library object you want to be able to place. Using the visua_model_guid= instruction in sim.cfg to call a library object for rendering.
Code:
[fltsim.0]
title=SceneContainer VEH_Air_PickupUS_Blue
model=
texture=
visual_model_guid={AAC710F6-68FB-4E7B-BF1D-458843B34C56}
ui_type=SceneryObject
[General]
category=SimpleObject
[contact_points]
destroy_on_impact=0
After you prepared such container files and eventually restarted FS, "live" placement
is possible.
I've exported a sim.cfg file for 3000+ objects of standard libraries and that solution works pretty well. I made a little placer using a top-down map projection to see and move objects from a remote computer and see the result immediately in FSX.
I used standard simconnect functions to maintain object position in FS, AICreateSimulatedObject/AIRemoveObject for inserting/deleting objects, SetDataOnSimObject to feed position change from the map application and TransmitClientEvent to set/unset slew mode (there are some weird things with setting altitude when in slew mode)
But there are also several quirks:
- the TRAN sections of objects called with this method are not taken in account (ie objects are always rendererd with an identity matrix)
- objects with animations are not rendered correctly when using visual_model_guid too. I believe this is just an outcome of the previous observation
- FSX is very unstable when the given guid does not name an object that can be found (I experienced several CTD due to this)
- fps impact. Much more sensitive than displaying the same object as a scenery element. I think a reasonable upper limit is 200-300 objects at the same time.
- While you can directly modify placement parameters (Lat Lon alt, PBH), the scale parameter of placement cannot be modified (always 1.0).
- No generic buildings, windsocks etc
When the FSX object has an animation, I extracted the mdl file from its library and changed the model= line in the container to use it. As you noticed, aircraft and scenery MDL files share the same format in FSX.
But for FS9 MDLs, i'm stuck with visual_model_guid. Although I've seen some old animated objects correctly rendered too.