Very high-risk move IMO. Personally, even as someone who will read and enjoy full-on crossover fics, I dislike these kinds of minor crossover cameos and find it throws me out of the story to have some rando from a different canon show up when it's not part of the main premise of the story I signed up for. (There's also a lower but non-zero risk of having picked a crossover fandom/character that the recipient will be more specifically annoyed by, especially if it's a well-known fandom where they're likely to have formed at least some opinion if only, "Ugh, can everyone please stop talking about X?") OTOH, some people do actively enjoy and get a kick out of Easter egg-type references and cameo characters! But I wouldn't risk it in a gift fic without at least a clear indication they're into and up for crossovers in general.