preferred overlay/union filesystem?

[Joe Pfeiffer <pfeiffer@cs.nmsu.edu>]

First, here's what I'm trying to do:

I'm using encfs to give myself an encrypted home directory, and I'm
successfully mounting it automatically using pam_mount when I log in.

My email is processed by a .procmailrc file in my home directory, and
I'm passing the email through bogofilter.  So, at present I have a
nearly empty home directory containing .procmailrc and .bogofilter/
which is accessed when I'm not logged in, and my "real" home directory
which is available when I am logged in.  This is, of course, less than
optimal since I have to keep two versions of .procmailrc and
.bogofilter/ in sync.

I'd like to have an overlay filesystem, so the unencrypted .procmailrc
and .bogofilter are still visible through a "hole" in my encrypted
filesystem.

I've been looking through the various union and overlay filesystems
(unionfs, unionfs-fuse, aufs, overlayfs....) and (1) I'm not sure which
of them are currently being developed and supported, and (2) it isn't
clear any of them actually do what I want (either I can't just overlay a
new filesystem on top of an existing one, or else the existing one has
to be read-only).

Any guidance would be appreciated!