I got fed up of using the Vocoder as a Spectrum Analyzer and so I've come up with the following solution that does not rely on ReWire. I'm not a Windows user (thankfully) but I'm sure this solution can be applied to that platform using similar Window utilities.
1. Firstly, you will need to install an awesome utility called Soundflower, made by the folks from cycling '74 and available as a free download. v1.5.2 is the latest version available. Soundflower is a MacOS system extension that allows applications to pass audio to other applications.
2. Secondly, you need an Analyzer! The one I've chosen is provided for free from the folks at Blue Cat. The analyzer you choose needs to be an Audio Unit (AU) plugin format. Here is the link for the Blue Cat analyzer.
3. Finally, we need a handy utility called "AU Lab" which is also free, but is part of the Apple Developer XCode installation. The XCode DVD is included with 10.6 and older OS X installation media. For Lion, and anyone else who no longer has their XCode media, you can download it from developer.apple.com.
AU Lab is intended for testing Audio Unit effects and instruments, and has a very high performance and low overhead audio engine capable of supporting multiple outputs, and advanced routing features such as send buses and sidechain buses.
1. Install your Analyzer AU plugin, along with you other AU plugins located in the Components folder, which is alised/symlinked for your convenience in the DMG.
2. Install the Soundflower DMG. Once installed fire up Soundflowerbed, open up the Menu Bar icon, and setup either the 2-channel or 16-channel device depending on how many channels your audio device has or how many channels you need.
Setting up the device is easy as selecting the Audio Device, and then selecting the channel routing.
Once you have setup your Soundflower device, go into Reason and use this device as your Audio Device. The signal path is now sent to the Soundflower device, and routed to your Audio Device so you should be hearing sound from Reason now.
3. Once you've completed those previous steps, you can now go into the final step which is configuring AU Lab.
Firstly open up "Au Lab", which can be found via Spotlight, and you'll see the configuration screen. In my example, you can see that I've selected my "Soundflower (2ch)" device for both the Input Source and Audio Output Device
Hit Create Document, and it will display the following screen with both Input and Output channels. Make sure you Mute the Output channel, otherwise you will get feedback from the Input channel (I'm not too sure why yet)
Almost there! The final step is to assign an effect to the Input channel, and we're going to be using the Blue Cat Analyzer AU plugin for that!
Your Analyzer plugin should be fired up now, and ready for the audio output from Reason. Give it a try!
Save your AU Lab config for easy reload later.