The Favorites subrelay could be used like that. It would require making the initial posts on some other relay & republishing there, but that would allow for collecting non-members' posts, too.

You could also simply not use the main relay as a read/write and publish exclusively to it, with the type of content that you want there. Members would just need to be in agreeance that it is a standalone community intended for one purpose. That's probably the right way to do it.