So, the trick to nailing easter egg cameos is first having their appearance make sense and using what I call pseudo-subtlety.
First, just have their appearance make sense. Chrono Trigger does this already with Biggs and Wedge who vanish out of thin air in Final Fantasy VI. Turns out they weren't disintegrated, they were transported. The game doesn't get into the specifics of how they got their. The Final Fantasy fans infer this knowledge from seeing their demise during Final Fantasy VI.
Another thing is their appearance didn't need to be modified to make them recognizable as that specific incarnation of the pair because CT's release was so close to FF6's. Their names were enough.
Which brings me to my second point about pseudo-subtlety. Unless, the character isn't both owned by Square and has a good reason to be their they should get either a name drop it custom graphics but not both.
Video games bend the 4th wall in this way all of the time. Fans have come to expect it. However, when they do so with characters outside of their IP, they never break this rule. While you aren't legally bound to this expectation, your players are used to it and any changes from that norm will feel out of place.
Take Doctor Who for example. A police box would fit fine on the street corner at the end of time. You could even have a suspicious looking bloke in a bowtie looking for Amy and Rory. But if he were to introduce himself "Hello, I'm the Doctor. Have you seen my friend's Amy and Rory around? I seen to have misplaced them." It suddenly feels forced.
However, let's see what happens when we see this unnamed character, bearing a distinct likeness to Matt Smith, pacing, scratching his chin,and muttering to himself, "Damnit Amy. Where have you and Rory ran off to now?"
Now, that feels lighter and more fresh right? It doesn't over-engage the player with needless details because it's not part of the plot. Just remember, unless there is an actual story arch that makes sense, a little cameo goes a looking way.