Most maintainers wouldn't want to be or couldn't be an employee of such a company (e.g. due to existing employment contracts). I'd also question whether it's healthy to centralize the development of all OSS projects used by all your customers into one platform like this.
That is fine, and there is no need for it. That wouldn't be viable anyway (except for node.js and projects of similar size).
The foundation pays someone else (their contractor) to fix a bug in the OSS project and to make a pull request to the project maintainer. This way the bugs get fixed, enterprise have their support and contractor gets paid. Maintainer of the project also gets something - fixes of his code by seasoned professionals, paid to do their work.
