I understand your pain soo much man. The way I did it with KF-Offices, is that I first made sure nothing was visible in the 3D view port, added the rain mesh, then duplicated while moving it upward or downwards, then replaced the mesh with something like a 2x2 modular floor mesh, then while holding ctrl+alt and draging the mouse made sure to put both meshes inside the box selection then grouped both meshes. So now you can just easily click on the floor mesh to select your rain mesh.This helps a lot but it's hard to pin point wether or not the rain drops will go through indoor buildings.
HOWEVER! You can do what came to my mind lately.
-Make sure you have 0 assets in the 3D viewport.
-View the rain mesh in its 'Static Mesh Editor' and you will be able to see the approximate dimension (L,W & H).
-Proceed to drag and drop the rain mesh (do not un-select it while it's placed in the level).
-Add the builder brush to the rain mesh (so the brush box remains in the center of the mesh.
-Scale the builder brush to the approximate dimensions of the rain mesh you viewed in the 'Mesh Editor'.
-Click the 'CSG add' button.
-Now convert the brush into a static mesh (Make the directory inside your level package and fill the other requirements to your liking)
-Then use the box selection method to select both the rain mesh and your box mesh, and GROUP them.
Now you can accurately move around the rain mesh and make sure it doesn't clip through rooms.
Do I get a cookie now? Make it a chocolate fudge.