Crossovers
Besides the usual fare (manga, movie, TV series, OAV), Seiichirou makes small appearances in a few other CLAMPish things.

"Tsubasa: RESEVoir CHRoNiCLE"
"Tsubasa" is the ultimate CLAMP crossover spree, the shounen version of "Card Captor Sakura" in which the Shaoran and Sakura of an alternate universe travel through worlds in search of the feathers of Sakura's lost memories. In each world they stop in they encounter a character or two from another CLAMP series in an alternative setting of their own. Seiichirou appears very briefly in Chapitre 41 in a country resembling Japan of the early 1900s, Outo. If you look closely at the page with the newsboy running down the street, you'll spot Seiichirou to the left giving the kid a sort of ! look. Look closer, and you can see Souhi and Hien in the background.

For more on "Tsubasa", check out Wing Journey.

CLAMP in Wonderland
This is a short, animated music video featuring flash appearances by members of various CLAMP series, including (but not limited to) "20 Mensou ni Onegai!!", "Mahou Kishi Rayearth", "Shin Shunkaden", "Tokyo Babylon", "Shirahime Shou", "Miyuki-chan in Wonderland" and, of course, "X". Aoki get a good 5 seconds of screen time, looking down at Tokyo from - where else? - Tokyo Tower. Karen is posing in the background, enjoying the nighttime breeze, and in the very far background you get a nice view of Yuuto and Satsuki's backs. Flash by a few generic looking hooded figures, and the Tokyo Tower lounging group is forced to flee when three giant mashin show up and threaten to crush them. If you've an eye for fleeting things you can catch Seiichirou jumping away. It's extremely cute, in a non-"X" like fashion.

X²
This is a 25 minute music video featuring four songs by the famous j-rock group X-Japan ("Kurenai", "Endless Rain", Silent Jealously", and "X: Hymn de Groupe"). From what I understand, the first three songs feature old school "X" color art. As I have not seen the first three parts, I cannot say whether or not Seiichirou made a cameo. Probably not, since I don't recall any color pictures of him dating from that time. The last part is a beautifully animated segment set to the song "X: Hymn de Groupe", featuring various scenes from the manga. Seiichirou and Arashi's first meeting was animated for this clip. Arashi fights off the men in black while Seiichirou watches quitely under his red umbrella.

Hajime - Soto