Mumble -- I would say that BSD portals was more in response to an MIT idea that the Apollo guys (Jim Rees I believe) put in domain as the Apollo Typed File System. At the time of the BSD, there was a lots of discussion going on about how to move things out the kernel and a number of techniques were considered -- Portals was one of them.
The idea of /dev/mumble/some_other_mumble was kicking around early UNIX for a long time. I think I first encountered it in the MIT Chaos code, but the Arpanet NCP code may have redated its use. The idea was simple, the kernel's nami() function that was called by kernel open code (or any other call the had to walk a pathname), left the pointer to the next undecoded part of the path in the user's input buffer alone. So a hack was put in the kernel that caused open to vector to needed helper code for the network to do the rest of the pathname to get the parameters. The advantage of this of course is open returns a real file descriptor.
BTW: One idea I had many years ago, and I remember talking to Dennis about it at a couple of USENIX was similar to Apollo's typed scheme. As dmr put it to me at the time, UNIX has a typed file system with very few types. The question was if we opened up the typing scheme to allow for arbitrary types, would you get more of mess or would the new feature be of an advantage. I was not doing research at the time - I was employed at Masscomp and just getting a distributed FS was enough. Since I did not have a good use for the hack for EFS, I never tried to implement it. It was always on my "TODO" list. Maybe when I retire and I can hack just for the fun of it, I can try it ;-)
Clem