Is there any other manga artist/s whose style is like CLAMP's?
Osamu Tezuka, perhaps ?

In his mangas, the same characters would appear in different roles. Given the ammount of work he produce, it's not a big surprise he could do that. Don't think it was intensive like CLAMP, but was well used, in specific situations.

Tezuka had his Star System, and his characters would appear in many stories, sometimes without a formal reference (not being identified as such).

Examples:

Black Jack made some apparitions in Astro Boy... Astro Boy made many apparitions in Black Jack and in other mangas.