Non-recursive spotlight search?

As far as I know, unfortunately it's not possible to do this.

Strangely, I also set the 'Document Container' criteria within Spotlight and it wouldn't show any results at all.

This is extremely poor behaviour, I agree. You can give Apple feedback on OS X (from which you can post feedback on Spotlight) here.

The command-line interface to Spotlight, mdfind, has no such capability. (There is an -onlyin flag but empirically it's recursive.)

Since it's extremely unlikely that the Finder would have more capability than the command-line interface, I think it's safe to say that this is not possible. Sorry. That does sound like it would be a very useful feature.

Not a solution for using the graphical interface, but i've started using the following in terminal to get the desired effect:

mdfind FOO | grep $PWD | grep -v ^$PWD/.*/.*$