How do I setup a floating profile in Linux

I have an external hard drive that is switched between machines.

I want my home folder to point to this external hard drive for ordinary usage purposes - documents, downloads, work areas, that sort of thing. For local system settings, such as local PATH, libraries, etc, it needs to support local machine settings. Programs should default to the floating profile for working directory (load/save), but for GUI setting and the like it should us the local machine settings.

I would like the GUI Desktop to "work" out of the floating profile, so if I save a document it will be on the external drive but will show up on my desktop normally.

I would like to sit down at the machine, plug in my external drive, have it auto-mounted to a known location, and then when I login through gdm or whatever, it should bring up the correct (external) desktop.

Ideally, if the external drive is not available (not mounted) I would like it to fall back to a sane/usable local configuration. It's okay if my external setting are not preserved but the profile should still be usable.

I am using MATE at present but could switch. I'd prefer to do this in MATE but if that's not possible, is there a GUI environment where this can be done? My distro is Mint 17 but I am thinking about trying Arch.

I've gotten the mounting stuff figured out, and am able to do some setup through my .bashrc. I tried editing .config/user-dirs.dirs to point to the external drive but it keeps showing files on the desktop instead of on the external drive.

I known this is a broad question, and it may not all be possible, but surely someone has trod this path before. Google is drawing a blank. Can anyone point me to a howto, an article, or share some tips on how to set this up?