_Any_ X server I know of communicates locally via Unix domain
sockets... about as close to pipes as you can get. (setenv DISPLAY :0.0)

And any X server newer than a few years supports the
MIT-SHM-Extension. It allows knowledgable clients which are on the
local machine two write directly into the graphics board RAM in a
controlled manner (_if_ the graphics board RAM is mapped into the
CPU's memory space). For example, mpeg_play does this.

Regards,
	-is