You can also just hide these by opening up the layers dialog and hide all layers called BLDNG (SOMETHING) and leave them hidden (unless you want demographics coloring).

Make sure city demographics colors are off:

City menu -> Layers -> All BLDNGs with shading

If that doesn't help, can you post the .fcw file so we can take a look at it?

You don't have to make symbol catalogs actually. CC3+ is capable of just opening a folder of .png files and displaying it as it was a symbol catalog.

Obviously, you lose all the features symbols catalogs give you, such as groups, random selections, forced sheets, and so on, but for a dynamic collection, the compromise might be acceptable, especially if they have a usable folder structure in this collection.

I do hope they only add items and don't modify/remove existing ones though, as that would wreck havoc with existing maps since CC3+ refers to the image on disk.

Making symbol catalogs for such a collection could actually be a nice community project. The symbols themselves would have to be downloaded from that site by the individual user, but common symbol catalogs wouldn't be a problem if someone could coordinate that and get updated ones out there as soon as the collection changes.

Not to mention the workflow of all the existing users who've learned to use it.

Unfortunately, it is the other way around. I don't think there is a way to stop the menus from interfering in a single command, and the alt key causes the menu to grab focus once you let go, not something that you would want in the middle of a command (bloody annoying basically)

Use layers. They are a great way to hide/show features. You can for example make one layer for each room, and then either have an entity on that layer that covers up the room, so hide the layer to show the room, or place everything in that room on that layer, so when you show the layer, the room appears.

Here is another blog article from me that may be relevant