Our (CDCK) multi-tenant hosting containers use an (internal-only) plugin to do what you describe – hard-code (or derive) default values for some settings, make settings invisible (and hence unavailable to be changed), that sort of thing. Programmatically creating new forums is simply a matter of creating (and migrating) a new DB and adding the config to the multisite config.
I wouldn’t call the whole setup “trivial”, but it’s certainly not the most complicated thing I’ve ever seen, either.