Yes, unfortunately CentOS does not provide the updateinfo metadata in their repositories.
I have not needed it in a while, but from different threads here I had the impression that it was working again with pulp 3. But I think @iballou will know this for sure.
But while the GUI has the workflow only implemented for erratas, using the API it is also possible for packages. So this could be a workaround if errata support for an OS is not available.
The more convenient way to handle emergency updates is with the incremental update feature. You can add errata or RPMs from main repositories to content view versions directly with it. It will create a “point-release” of the content view version with your added content.