For a series well-known for having guest characters, Soul Calibur hasn't really crossed over with other series that much for some reason. The only notable guest appearance I'm aware of is Sophitia being in a Warriors game, which is frankly kind of a low bar, honestly. Of course, the lack of previous crossovers doesn't really affect a character's chances, but it's still something I find abnormal for a series as big as Soul Calibur.
Sorry, for a late response and if you are still wondering in terms of
Soul Calibur characters or references appearing in other crossovers.
Here are some answers:
Pac-Man Fever: Astaroth
Warriors Orochi 3 Ultimate: Sophitia
Namco x Capcom: Mitsurugi, Taki, Charade
Project x Zone 2: Natsu
Point Blank: Yoshimitsu (cameo)
Ridge Racer references: Astaroth (tires/suspension manufacturer), Kilik (exterior parts manufacturer)
Tales series: Celsius Calibur = Soul Calibur (weapon), Mitsurugi outfit in Vesperia (DLC) and Taki outfit in Radiant Mythology 2.
Pro Baseball Famista 2011: Mitsurugi, Siegfried
Famista Dream Match: Mitsurugi, Siegfried, Taki, Ivy, Astaroth
Famista Climax: Mitsurugi
Smash Court Tennis Pro Tournament 2: Cassandra, Raphael
Queen's Gate/Queen's Gate: Spirial Chaos: Ivy, Pyrrha, Taki
Little Big Planet outfits: Nightmare, Cervantes, Ivy, Sophitia, Taki, Voldo.
Outcast Odyssey (trading card game): Sophitia, Siegfried, Nightmare, Tira, Voldo, Kilik, Elysium, Maxi, Ivy, Mitsurugi, Astaroth.
Universal Fighting System (trading card game): Mostly based on Soul Calibur III and IV; Inferno, Cervantes, Raphael, Talim, Astaroth, Seong, Tira, Voldo, Groh, Ivy, Kilik, Maxi, Mitsurugi, Nightmare, Siegfried, Sophitia, Taki, Xianghua, Yoshimitsu, Zasalamel, Amy, Azwel, Cassandra.