Assuming a fake Cloak, I suppose Flamel could have programmed the Mirror with contingencies to trigger on “anyone wearing the Fake Cloak of Invisibility”, rather than on “anyone reflected in the Mirror” as we originally expected.
Edit: Actually, that might not work for the same reason that putting mayonnaise on a hamster might not work: the fake Cloak isn’t inherent to the target, and so the Mirror doesn’t care about it.
Assuming a fake Cloak, I suppose Flamel could have programmed the Mirror with contingencies to trigger on “anyone wearing the Fake Cloak of Invisibility”, rather than on “anyone reflected in the Mirror” as we originally expected.
Edit: Actually, that might not work for the same reason that putting mayonnaise on a hamster might not work: the fake Cloak isn’t inherent to the target, and so the Mirror doesn’t care about it.