Indeed. It seems there are many such tools available for MySQL: Openark kit [0] for instance, whose author also brought us gh-ost. Unfortunately most tools focus on just MySQL, and none really have an answer when you want to use (and enforce) foreign key constraints. Although it's hard to say, I suspect the users of these tools have "given up" on foreign key constraints.
[0] http://code.openark.org/forge/openark-kit [1] https://github.com/github/gh-ost