It's been a long time since I've used FIT, but I seem to remember that
it was simply a device tree-based description of a file format. As such
couldn't you add an additional property to the configuration entry that
the bootloader could match on? That would avoid the need to parse an
embedded device tree and it might be useful to keep variants for the
same board in the same FIT image.

Yes, I suppose something like that would be possible, however it's not
just depthcharge that matches to configurations based on the
compatible string within the configuration's FDT blob, this is the
convention U-Boot uses as well when booting FIT images.

Perhaps you can explain a little more what the use-cases are. For
example how are people suppose to run an upstream kernel on one of these
Chromebooks?

There are a few ways you could run an upstream kernel:

1) Building your own verified-boot-wrapped FIT image and replacing the
existing one using tools found in the ChromiumOS tree [0].  This is
the workflow must of our developers use when working on product
kernels, but could just as easily be done with an upstream kernel.

2) Using U-Boot, either through the built-in legacy mode payload
(Ctrl-L at the developer screen) or by replacing the vboot-wrapped FIT
image with one that has U-Boot as a payload rather than a kernel [1].

3) If you're willing to flash your own firmware to the device, you can
build netboot-enabled firmware form the ChromiumOS tree that will load
a FIT image over the network.  This is the most flexible, as it
doesn't require the root device to be partitioned in any particular
way and allows you to boot non-ChromeOS distributions (e.g. Ubuntu).
This is what Dylan and I usually do when working on upstream kernels.

How does dual-booting work?

You could dual-boot with the legacy-mode U-Boot, described above.

Does the user get a menu to choose a configuration they want to boot if
multiple entries are compatible with the detected board?

IIRC depthcharge will just pick the first one if there are multiple
exact matches.

[0] http://www.chromium.org/chromium-os/how-tos-and-troubleshooting/kernel-faq#TOC-How-to-quickly-test-kernel-modifications-the-fast-way-
[1] http://www.chromium.org/chromium-os/u-boot-porting-guide/using-nv-u-boot-on-the-samsung-arm-chromebook#TOC-Installing-nv-U-Boot-chained-U-Boot-method-