Idea: option to create only instances specified in config, ignoring designspace instances

As the title says, I'd like to have a mechanism to build static instances, but only those specified in my `config.yaml`, not the ones in the designspace source I'm using. Currently, both are always built AFAICT

I wouldn't mind either a commandline flag or a config file option

Would such a thing be doable, or is this too niche to be considered?