There seems no way yet to include custom Xcode configuration with a Projucer project. Since xcconfig files support #include, they are great for uniformly maintaining multiple projects with a shared configuration base (version numbers, build locations, codesign signatures, deployment targets, stripping options, you name it).
The field Custom Xcode flags doesn’t work for including a xcconfig file (unless I missed something). It would be fantastic if there was a field Include Xcode config file instead. This would allow for arbitrarily complex build environments to nicely cooperate with Projucer.