Catching the vtable updates and propagating them downwards is pretty simple, you "just" need every class to know which classes inherits from them. There's an implementation for dynamic runtime updates of dispatch tables for Oberon, of all languages (though that version sidesteps the "sparse vtables" issue by splitting the vtables into interfaces, and adding one extra level of indirection).

The tricky bit is if you have gone as far as inlining the method.