There are plenty of existing libraries that do exactly that. This isn't novel to Fuchsia. A good example is GNOME's GVfs https://en.wikipedia.org/wiki/GVfs , which is basically a plugin architecture to the standard GLib I/O routines. (Although as it happens, it still places the mounts in separate daemon processes.)

Other things that come to mind are SQLite's VFS layer https://www.sqlite.org/vfs.html , Apache Commons VFS for Java https://commons.apache.org/proper/commons-vfs/ , glibc's fopencookie(3) which lets you provide a custom, in-process implementation of a FILE * http://man7.org/linux/man-pages/man3/fopencookie.3.html , libnfs which even comes with an LD_PRELOAD https://github.com/sahlberg/libnfs , etc.

(And as others have pointed out, while client and server code can exist without IPC, as the names "client" and "server" would imply, that isn't the primary intention. The docs you link say, "To open a file, Fuchsia programs (clients) send RPC requests to filesystem servers ...." And even the terminology of a file system as a "server" isn't novel to Fuchsia; that's the approach the HURD and Plan 9 both take for filesystems, for instance.)