The problem is that adding FK constraints is another schema change, which causes MySQL to copy the whole table, and lock the table during this time -- which is precisely what gh-ost tries to avoid.
Worse, foreign keys from other tables to the one that is being changed would need to be updated as well, blocking those tables in turn.
Worse, foreign keys from other tables to the one that is being changed would need to be updated as well, blocking those tables in turn.