Same as the previous post I mentioned, you just need to precache these ents prior to players joining, at OnMapStart. No need to strip them out. Just create one at 0,0,0 and the server will call the proper precache materials for it.
I normally delay it until the first player spawn (I have bots) so I am sure its not going to be too soon and fail on the entity creation.
My post was to point out that some maps use sprites that do bot exist in the game etc so you could precache that material anyways