Appart from the trigger event, is it
what you had in mind ? Am I querying
the right class and components ?
Yes that is what I meant. You can then access the static mesh for each HISMC and compare it with those you’ve shown in your image trees-content.png to see whether that HISMC uses that static mesh. And you can access the transform (location, rotation, scale) for each of those instances (e. g. if the static mesh would be SM_RW_SQM1 then each instance would be a tree of with SM_RW_SQM1) via GetInstanceTransform. So with that you can get the transforms for all your trees.
Of course you can only access the instances that way (and since you access the HISMC via the InstancedFoliageActor only those that are in there, so what you place via Procedural Foliage Spawner / with the foliage paint tool). You can’t access static mesh actors that way. If you just drag and drop a static mesh into the world, then you will get a StaticMeshActor. So you cannot access anything of your world-outliner-river.png using that way.
If you want to access the transform for static mesh actors that use a specific static mesh, then you can use GetAllActorsOfClass with StaticMeshActor and then check what static mesh the static mesh component of the StaticMeshActor uses. The transform would then just be the actors transform.