I may be mistaken,but my understanding is that userspace servers accessible through the use of the fopen/fread/fwrite, etc. system calls was an idea that originated in Plan 9.
Furthermore, some people who worked at Bell Labs at the time now work at Google.
http://multicians.org/rjf.html
Now, the question is when was same concept applied to user-space servers? It was possibly Mach that was UNIX-like with all processes communicating through ports that were like standardized pipes.
https://en.wikipedia.org/wiki/Mach_(kernel)
You got user-space components to open them, read them, write them, etc. That was 1985 whereas Plan 9 hit universities in 1992. Both models had security and performance issues that led reliability- and security-oriented OS's to go different routes on purpose. Plan 9 was a step up from UNIX but a step down from high-assurance (eg KeyKOS, NonStop) and high-flexibility systems (eg SPIN OS, LISP machines).