I believe the /proc placeholder directory is just a linuxism from some of the base debian packages, so do not draw any conclusions from its existence.
There are no particular plans to implement a /proc filesystem. If you would like to write one, more power to you. Indeed, "translator" and "filesystem" are somewhat interchangeable terms in the Hurd and what you would be writing is certainly a translator. To start with, you need get a feel for writing a translator. The files trans/hello.c and trans/hello-mt.c in the Hurd sources are trivial "hello world" translators (a single-threaded version and a multi-threaded) to give you a start at the basic idea. There is substantially more work (and less code to easily guide you) involved in a translator that provides directories rather than a single file node. The other aspects of the work are how you get at all the information you might want. This information (what is "process state" in Linux) is divided into four pieces in the Hurd: virtual memory, threads, and POSIX process state maintained in the proc server, and POSIX process state maintained in each process by the C library. The virtual memory and threads are provided by the Mach microkernel, and along with the IPC state, these form a Mach task. (There is a one-to-one correspondence between Mach tasks and Hurd processes.) The `vminfo' program (source in utils/vminfo.c) will give you a feel for what a process's virtual memory state looks like. The job of providing a mappable file interface for a task's address space is already done by the `task' store type (source in libstore/task.c), which would be used by a translator like storeio; but I don't think that has really been tested, so it might be broken. Anyway, that's about the least complicated piece of the puzzle (in that it's a whole huge world of complication that is already mostly handled elsewhere). All of the other information I mentioned is already accessed by `ps', and the code that does that is in the `libps' library. You might want to just use that, depending what you want to do with all the information. Reading that library should give you an idea where all the pieces come from and how they are acquired. (Run ps with different options and format strings on various processes to get a feel for what each datum means, and then go look in the libps source code to see what the data structure for that looks like and how libps finds the information.)
