It would be simple matter of having two software support licenses. One for operating system and one for application. This is how most software works today.

Red Hat knows if their customers are buying Red Hat license to get support for LibreOffice. I would assume this is a case only for very small number of customers, as Red Hat is mostly focusing on server product. And today companies can buy the same support from LibreOffice directly, so there is no point for Red Hat to compete against LibreOffice here. It's win win.