Author of gh-ost here. Actually, it should be possible to support child-side foreign keys. They would have to be named differently (the foreign key constraint has a unique name in a schema) -- but it should work. See discussion in https://github.com/github/gh-ost/issues/507

It is true that it is not on our roadmap to implement FK support for gh-ost (see https://github.com/github/gh-ost/issues/331), but if anyone wishes to contribute support for FK we're grateful. We've had more complex contributions coming from the community and we're grateful for those.