But the server and client are using the same folders! It's the same binary, as I said above.
If we use different folders for the two, then we need to reship all .wrs files again a second time, which will almost double the file size.
this would happen every round-restart and would slow down the loading process even more instead of speeding it up, I believe.

I know. What I meant is, that you send only the wrs with the dummy textures seperated from the game folders within the zip.
The dummy textures are supposed to be tiny.
Maybe, you can include two installers, one for client mode and one server mode, where in server mode the standard texture wrs will be replaced by the dummy textures. No different binary needed. Means: in server mode there are just different textures in the same folder.

Or: even better. Maybe, you can implement in the code that when starting as server the engine loads the textures from the dummy texture folder, and when starting as client load them from the standard folder - I'm not sure whether this is possible, though.

No, I don't think it can be done by code. That's why I asked here, and since jcl did not reply with "use code to load the right wrs files depending on whether you run as server or client" I assume it's not possible. Also, the manual's pretty clear on this:

That's usually very good, but in my case makes things difficult.

add_folder/add_resource is what you're looking for

I can hardly imagine that the slightly shorter loading time of smaller textures is worth the hassle. How many seconds do you gain on level loading?

It's not the level loading time I'm worried about, I want the game to use less RAM. And I think there, the difference would be noticeable (but I must admit I have no tests done to back that up with).