Seems like if you symlink the relevant SDK from a previous installation of Xcode to the Xcode 6 Beta directory, Xcode 6 will be able to use it. Assuming you still have Xcode 5.1.1 on your machine, try the following in terminal:
ln -s /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS7.1.sdk /Applications/Xcode6-Beta.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS7.1.sdk
This should link the iOS 7 SDK for iPhone (not sim), allowing you to select iOS 7 as the Base SDK in Xcode 6 Beta. If you want to do this for the simulator, replace iPhoneOS.platform with iPhoneSimulator.platform.
NOTE: Just because you have the iOS 7 SDK now doesn’t mean it’ll build. I’m personally having trouble with the fact that Xcode 6 Beta is using the Apple LLVM 6.0 compiler, which apparently isn’t compatible with some of the stuff I’m trying to build.
ref: https://stackoverflow.com/a/11424966/2383003