Specifying a subproject's Configuration in Xcode

I don't know of any easy approach, but you can brute-force it by calling xcodebuild directly for the dependency with a "Run Script" build phase.

I know it was just an example, but if your real goal is that the sub-project be a Release (no symbols) build, then you may have a better experience by just building the sub-project into a library or framework and checking the resulting binary into your version control system. Whenever I have a piece of the system that seldom changes and that I don't want debug symbols for, I go ahead and build it as a static library and check it in. I often go ahead and move the code elsewhere as well (with a README file with the .a that says where the code is and how it was built). This saves time on both build and checkout and is invaluable for large projects in my experience.

This might help: if the configuration of the project A is not found, Xcode will build Release config as a fallback (or maybe the first config of the list).

Then you can "force" the link using this tip: Xcode custom build configuration causes "library/file not found" for static libraries

Yes, this is not naturally supported by Xcode; when you build a target, it builds one configuration of itself and of all dependent targets.

The workaround, as Rob mentioned, is to have a dependent target that's an Aggregate Target type that comprises a single Run Script build phase, which simply invokes xcodebuild -configuration Release (or whatever).